the world's most famous spy organization, the legendary home of Agent 007...James Bond.

This is MI6, otherwise known as Her Majesty's Secret Service.

Now, with unprecedented access to the major players in British espionage...

It's exciting and the adrenaline is pumping round.

..we reveal the truth behind the covert operations, the secret missions and the double-agents.

We expose the dangerous secret world of real-life spies.

There are far more spies in London now than there were during the Cold War.

This program is made possible London, a cosmopolitan metropolis of 8 million inhabitants.

And home to the world-renowned British Secret Service.

For decades, cunning games of espionage have been played out on the city's streets.

Assassinations... treason...and betrayal.

London has always been a crossroads, a global city.

You've always had lots of exiles and dissidents here.

That means it's a real meeting place for spies - British spies, spies from other countries trying to recruit people, trying to run operations.

I think it's meant that London is one of the great spy cities in the world.

Entering the high-stakes world of spying, we learn what's really going on in the shadows.

From the highest echelons of MI6...

The best form of defense against plots and attacks is to know about it in advance.

It was all about collecting intelligence and recruiting and running sources.

..to field operatives who risked their lives... At my peak, I was operating under about 14 or 15 different aliases.

..unsung heroes of Britain's most secret code-breaking station... We broke a message that said 'Today's the day minus three.'

..the KGB double agents who fought the Cold War.

The Secret Intelligence Service exists to defend and protect the security and interests of the United Kingdom.

It's known by its World War One designation as Military Intelligence Section 6... or MI6.

This green and cream building behind me is the headquarters of MI6.

That's the Secret Intelligence Service, Britain's foreign intelligence service, the equivalent of the CIA.

Within intelligence circles, the building is known as VX, after its location at Vauxhall Cross on the banks of the River Thames.

It became even more identifiable to millions of cinema fans around the globe when it was famously blown up in the bond movie, Skyfall.

It is probably the most famous secret service building in the world thanks to the James Bond films.

The fictional world of 007 may be glamorous and entertaining but the reality of life in MI6 is far more serious.

Someone who knows this dark world better than most is Sir John Scarlett.

For 38 years, he was one of MI6's top operatives, rising to Chief of the Service in 2004.

MI6 is essentially an information gathering agency.

It collects intelligence.

That is its job and it is very good at it.

During his five-year tenure as Chief, Sir John authorized many dangerous and daring intelligence operations.

There's not much I can share but I think, like many of my colleagues, over a long period of time, I've quite often been in situations where it's exciting, and the best way of describing it is, the adrenaline, you know, is pumping round.

To preserve secrecy and security, MI6 never acknowledges its missions or spies.

But one spy has outed himself and talked - former undercover MI6 officer and now author, Matthew Dunn.

The majority of what I did was recruiting foreign spies, typically nationals who were placed in positions that gave them access to secrets and targeting senior echelons within rogue states.

That was the core of my work.

In MI6-speak, those passing on secret intelligence are called agents, while men such as Dunn, who run the agents, are called officers.

Both are commonly referred to as spies.

Dunn typically met his agents in the bars and rooms of London's top hotels, glossy surroundings that are far more dangerous than they seem to the average tourist.

The biggest concern a case officer has when attending a covert meeting with an agent is fear that he is being followed by a hostile surveillance team.

In order to address that, the officer will walk a anti-surveillance route.

Officers will establish what's called a theme, and in this case, I'm walking down Jermyn Street and my theme is shirt shopping.

If a team's following me, they will think I am shirt shopping.

If an officer spots the same face more than once on his anti-surveillance route, this is called a double-sighting.

This person may well be enemy surveillance, in which case, the officer has to abort the meeting.

I've often felt when operating that there was a threat to me, that I could be snatched or the person you're with could be snatched.

And that threat could include death.

Today MI6 has over 3,200 officers operating across the globe.

They run covert operations, infiltrate terrorist organizations and foil international plots.

They need to extract information from the most sensitive of sources.

One of the core strengths of MI6 is our agent-running skills and that is world renowned.

Other agencies - the CIA, European agencies, do recognize that our agent-handling abilities are second to none.

MI6 will use any tool in its arsenal, including blackmail and bribery, to persuade foreign agents to betray their country and hand over top secret information.

This is what happens in this sort of business.

That was a great skill.

You have to talent spot people in the first place, you have to spot them, see their potential and then bring them under control.

These skills have been perfected over the agency's 100-year history.

Britain's Secret Service was founded on 1st October, 1909, to gather intelligence about the growing military threat from imperial Germany.

Tensions were rising in Europe and people feared German spies were infiltrating Britain.

The Daily Mail told its readers to refuse to be served by German waiters: 'If your waiter says he's Swiss, demand to see his passport.'

MI6's first headquarters were near the corridors of power in Whitehall.

But few in government knew the agency even existed.

Where we are now is where the earliest headquarters of MI6 is, and that's up in that tower.

2 Whitehall Court was the foreign section of the Secret Service Bureau.

It's where Mansfield Cumming set up his early headquarters and throughout the First World War, that's where what we now call MI6 was based.

Mansfield Cumming was the first Chief of the Service.

He was a naval officer who would go on spying missions wearing rather bizarre disguises.

His office was a laboratory of strange gadgets and devices.

He was very innovative, he was one of the first people in Britain to take his pilot's license.

He had a special license plate for his Rolls Royce which allowed him to drive up and down London streets at any speed he wanted to without being stopped by the police.

He was a real character.

His staffing and interview methods were uniquely gruesome.

He had a wooden leg.

And when he was recruiting new officers, he would frequently take his pen knife and stab it into this wooden leg.

If the officer didn't flinch, he was just the right man to work for Britain's Secret Service.

Cumming knew that good agents were the most important tool in the spying game.

From the very beginning, he understood it was all about collecting intelligence and recruiting and running sources.

From the very beginning, that's what he did.

It was just in the bloodstream.

Cumming disguised his organization's true activity by pretending it was an import/export business called Rasen, Falcon & Co. To this day, MI6 uses fake companies as a cover for its clandestine operations abroad.

Cumming also created other traditions still maintained by his MI6 successors.

He brought with him the naval tradition of the captain writing his log in green ink.

Even now, the Chief of MI6 still writes in green ink, because Cumming did.

His most famous legacy, however, is his signature.

Cumming was given the title of C as a cover, it was simply the first letter of his name.

It's obviously where M in James Bond comes from and even now, the Chief of MI6, as he's now called, signs his letter with a C. Signing yourself as C is a reminder that we have a continuing tradition.

Everything leads to everything else.

There's been no break.

That's what's exceptional.

In 1918, Cumming recruited his most brilliant agent, the infamous Sidney Reilly, Ace of Spies.

Reilly, who could speak seven languages fluently, was utterly fearless, a hardened gambler and a famous seducer of women.

In some ways he was the perfect spy because he was a bit of conman.

And he was able to persuade people to do things, just like any conman, that they wouldn't normally do.

So he is a great collector of intelligence and he has these daring operations.

Reilly's adventures are straight out of the pages of a novel.

In France, he poses as a priest to win Britain important oil concessions.

In the Far East, he steals Russian naval plans to sell to the highest bidder.

He was operating as a film agent and as a Greek businessman, changing his identity at various stages.

After his death, the Evening Standard Newspaper serializes his memoirs.

He is at once immortalized as a master of deception.

He was the Ace of Spies.

He was brilliant.

In 1926, MI6 moves to new headquarters opposite St James's Park underground station in London.

It grows quickly into a sophisticated organization, collecting intelligence, running agents and mounting covert operations overseas.

Within 15 years, it has a spy network stretching across the globe.

It won't be long before the organization's capabilities are tested to their limits.

'The German foe begins its ruthless march of conquest.'

On 1st September, 1939, Germany invades Poland.

Europe is at war.

Now more than ever, Britain's Secret Service needs to know what its enemy is up to.

Europe's freedom depends on it.

One of MI6's new recruits is Mavis Batey, a 19-year-old student studying German literature.

Little does she know what lies in store.

I was told that I was wanted at a place in the country.

They wouldn't tell you where you were going but they told you to go to Euston station and you would be met with a ticket.

I thought I was going to be Mata Hari, I was going to be a spy, I was terribly excited.

Mavis's mystery destination is Bletchley Park, an old manor house hidden 50 miles north of London.

It's the home of a top secret department within MI6 that is decrypting enemy coded communications.

(Static whine) Mavis's boss, chief cryptographer Dilly Knox, is an eccentric chain-smoking classics scholar.

He was sitting, smoke all round him and he just looked up and said, 'We're breaking machines.

Have you got a pencil?'

And that was my introduction to breaking Enigma.

In 1939, Enigma is the world's most secure encryption machine and the Germans are using it to encode all their war communications.

The machine uses a clever combination of interchangeable rotors, moving rings and switchable plugs to scramble its messages.

The Germans believe its codes to be unbreakable.

The number of ways the operator could set this up was roughly 158.9 million million million.

The daily setting was known as the key and the setting changed every day at midnight Greenwich Mean Time.

Breaking the Enigma codes will be key to winning the war.

Mathematicians may be an obvious choice for the work but the unsung heroes of Bletchley were a unique group of young women.

There was one actress, one who'd been to a drama school, and of all things, a speech therapist.

And they were all people who knew patterns of language and picking it apart.

Armed with only pencils, paper and bits of card, it takes the code breakers five months of ceaseless work to decipher their first Enigma message.

Mavis's big moment would come a year later during a critical period of the war.

The British learn the enemy are planning a surprise attack on the Allied fleet in the Mediterranean.

But they don't know when it's coming.

Then, on 25th March, 1941, Mavis decodes an extraordinary message.

We broke a message that said, 'Today's the day minus three'.

We then knew what they were planning.

Everyone was so excited about that.

Forewarned of the impending offensive, the British call in backup and turn the tables on the enemy.

Thousands of lives are saved thanks to Mavis and her colleagues.

Well, we were very good at it, I suppose, very cunning.

Because it was largely a strange mixture of logic and guesswork by which you broke things.

So we were all a bit dotty afterwards and very good at Scrabble, of course.

Churchill considered the code breakers' work so vital to the war effort that he famously called them 'the geese that laid the golden egg'.

It is extremely unusual to get such comprehensive... in fact unprecedented, in my experience.. to get such comprehensive coverage of your opponent, especially in a war, both at the operational level and also at the level of their top-level thinking and strategic intentions.

As the number of enemy communications increases, Bletchley Park lacks the money and manpower to decipher all of them.

The only people in a position to help are the Americans.

So in February, 1941, a secret delegation from the USA is invited to Bletchley Park for an historic meeting.

Britain reveals its most guarded wartime secret, the cracking of the Enigma codes.

But it has an ulterior motive: to draw the US into the war.

By bringing the Americans across, by confiding in them about breaking the German codes, you try to make them partners in the effort, even though the Americans are not formally in the war.

You try to give the idea that somehow we are both in this together against Hitler.

This is the first time that any two nations have agreed to share such vital secret intelligence.

It's the beginning of what will become known as 'the special relationship'.

At that particular time, we had no formal treaty alliance between us.

There wasn't the history of co-operation which subsequently developed and if you're looking for a sort of single symbolic event which captures that rather remarkable situation, then it is here we were, prepared to talk to each other about the most secret things that we were doing.

The relationship gradually deepens as the two allies work together.

Eventually American code breakers are even stationed alongside the British at Bletchley.

The Americans were absolutely fascinated because when they came and saw us with these things in jam jars, they just couldn't believe what we were doing.

One of them said, 'My goodness, if they were the Pentagon, there'd be rows and rows of shiny files and nothing in them, and you do everything in these goddamn shoeboxes.'

The methods might be basic but the Americans are still impressed by the Brits' ingenuity.

They recognize how well MI6 coordinates all its various sections from its London headquarters.

Everything from cryptography to counter-espionage is under this central control.

The US has nothing like it.

In the words of Henry Stimson, who was the Secretary of Defense in the US, 'Gentlemen don't read each other's mail'.

So the Americans didn't see the need for a peacetime intelligence service.

In 1942, the Office of Strategic Services, America's very first centralized intelligence organization, is set up at Rockefeller Center in New York City.

Heading the agency is former army colonel William Donovan.

Because he was an anglophile, he said, 'We'll structure it the way the British have set up MI6.

We'll set it up for intelligence, analysis and also covert operations.'

British brains to guide American brawn.

The British let the Americans use their espionage network and covert listening stations across Europe.

The Americans are rapidly trying to catch up with the British.

So we have the development of a special relationship of structures but we also have the development of a special relationship of people, people who get to know each other during the war and who, after the war ends in 1945, continue to be in contact with each other.

By 1947, the OSS is reformed as the world-renowned CIA.

American co-operation with British Intelligence continues to grow closer.

That initial move lies at the very heart of the quite unique defense and security relationship that the United Kingdom has had with the United States ever since 1941.

In these early days of the war, Churchill sees that he'll need to fight dirty to stand any chance of winning.

MI6 runs secret sabotage missions abroad from a small unit that is aptly named 'the Destruction department'.

In July, 1940, they join forces with government propaganda and research departments to create the Special Operations Executive.

Its mission is to sabotage the German Army and set Europe ablaze.

To do this, it adopts the guerrilla warfare tactics used so successfully by the IRA.

This secret army has its beginnings in the most unlikely of elegant and refined surroundings.

Author Gill Bennett has documented official government history for more than 30 years.

Well, this is the St. Ermin's hotel, which has definitely strong intelligence connections particularly from the Second World War.

It's only round the corner from the headquarters of the Secret Intelligence Service at the time, SIS, very near to Whitehall, very near to Westminster, and so you could just go across St James's Park to wherever Churchill was, to what are now called the Cabinet War rooms.

Part of the Special Operations Executive, SOE, was formed here, it had its offices upstairs.

It looks very beautiful and light and romantic but it was nevertheless a place where a lot of clandestine meetings took place.

Most SOE operatives are foreigners who have fled occupied Europe.

French, Spaniards, Danes and Poles are recruited for their language skills and knowledge of occupied territories.

All are united by their desire to liberate their homeland from the Germans.

They were often called a bunch of amateurs.

Actually, a lot of them were a bunch of amateurs but it was not a universally liked part of the intelligence establishment, even though, of course, in retrospect it is seen as having had a major impact.

Like any other MI6 operative, preparation is key to their survival.

Before being parachuted into enemy territory, SOE agents undergo months of intense commando training.

They would learn how to do self-defense, silent killing using their bare hands, and also close combat, using a variety of weapons.

Machine guns, daggers, silk escape maps and even cleverly disguised suitcase radios are all part of their spy kit.

This is a Welrod pistol.

This is a silenced weapon, so it's very, very effective for assassination when you need things to be quieter.

It's a very iconic SOE weapon.

Women would be seen out wearing their hats all of the time.

So take your hat pin, make it thicker, stronger and with a very, very sharp point.

And as long as you know where you're aiming for, this dagger is going to be extremely effective.

SOE used plastic explosive.

This is a very malleable type of explosive.

A famous SOE weapon is the rat.

Get a desiccated rat, scoop out its insides, stuff it with plastic explosive.

A German sentry in a factory would think nothing of kicking aside a dead rat.

The explosive will blow up the entire factory.

(Gunfire) The SOE are hugely successful.

13,000 agents rally over a million local resistance fighters to cause chaos for the German occupiers.

On the night of D-day itself in France alone, there were 960 acts of sabotage carried out by the French Resistance against rail networks and on the roads to prevent Germans moving forwards towards Normandy.

After the war, Ian Fleming is inspired by the daring exploits of SOE and MI6 and uses them as plotlines for the elaborate missions of his master spy James Bond.

The James Bond stories are uniquely influential in defining the image of MI6 and its spies.

They sell the public a world of espionage that is glamorous and sexy.

Exotic cocktails, shaken not stirred.

Beautiful women at every turn.

And sharp-suited spies.

Fleming's fictional version of British espionage is so convincing, the Egyptian Secret Service are completely taken in by it.

Actually in the 1950s, the Egyptians believed everything that was in the James bond book must be based on reality.

The Egyptian Secret Service had their man in London buy up copies of every one of the James Bond books so they could analyze it and try and work out what bits were real and what bits weren't real.

Even today, MI6 is inextricable from the Bond mythology.

Much of the public's knowledge of the Secret Service comes solely from Fleming's creation.

MI6, which is a secret organization, also happens to be one of the most famous organizations in the world.

But it's the result, obviously, of the literature.

And that can bring problems, because people sort of misunderstand what intelligence work is all about.

I was in service for a long time and I had plenty of exciting moments... ..but they weren't James Bond moments.

The reality is that espionage work is neither loud nor high-profile.

For MI6, discretion and secrecy are key.

Unlike James Bond, I wasn't able to walk into a swanky bar in a hotel and announce that I was Matthew Dunn and have women flocking all around me.

We want to work in the shadows.

We don't want the flash-bang aspect of a James Bond movie.

As an MI6 officer, Matthew Dunn was trained in unarmed combat, explosives, firearms, exfiltration and a host of other intelligence techniques.

He's had to use all these skills at one time or another.

Everything I did was under what we call alias, assumed identity.

That assumed identity is not just a name, it would mean you'd have a passport, bank details.

You'd have to have in here a complete background to that alias identity.

You could be stopped going through customs just randomly.

So if you're taken into an interview room you've got to be able to convince the person opposite that you are the person you say you are.

That can include which schools you went to, what's your local pub... you know, small minutiae that is actually very important.

That's what I did and at peak I was operating under about 14 or 15 different aliases.

Dunn was recruited from university.

For most of its history, the British Secret Service has recruited its spies from the upper classes.

It's an elitism which would prove a critical mistake for MI6 and cost the lives of countless British agents.

It would also severely damage Britain's special relationship with the US.

In the 1930s and '40s, exclusive St James's clubs such as White's, Brooks's and Boodles are renowned for their links with British Intelligence.

Well, this is clubland.

This is what we call clubland.

It's just off Piccadilly, Buckingham Palace is just over there.

It's the heart of the Establishment at this period in the early 20th century.

This was the recruiting ground for MI6.

This was also the high society hangout for James Bond's creator Ian Fleming.

He incorporated his favorite clubs and upper class bars into the 007 stories.

Right, well, this is Boodles.

This is, essentially, James Bond's club.

In the books, it's called Blades but Ian Fleming was a member of Boodles and he used Boodles as a backdrop, if you like, for the Bond books.

Officers would conduct secret meetings behind the closed shutters of these private clubs.

Even today, members of the intelligence community use these private spaces.

This is the Reform, this is one of these hallowed clubs, places where gentlemen used to gather.

You can imagine people meeting behind these pillars or over lunch in the restaurant.

Because they were from the Establishment, they were regarded as people who could be trusted.

They were gentlemen.

They knew what they were doing.

In the 1930s, the KGB realize that they can take advantage of the cozy relationship between the British Secret Service and the British upper classes.

They begin to recruit Englishmen willing to betray their country.

If someone said you were the right kind of person, a good chap, then you were in.

There were no more security checks than that, no one bothered to look into your background.

People said, 'He's one of us.'

They were wrong and the Soviet Union very, very effectively exploited this by recruiting people and turning them against the British Establishment.

Cambridge University was one of the KGB's most successful hunting grounds for potential British traitors.

And the most famous of all was Kim Philby.

Kim Philby became converted to Communism in Cambridge in the early 1930s.

He believed that the choice the world faced was between Fascism on the one hand or Communism on the other and he chose Communism.

Philby was instructed by the KGB to join the British Secret Service and then to work his way up into a position of great influence - all the time passing secrets to his Russian contacts.

He proved remarkably effective.

He was the liaison with Washington at the start of the Cold War, where he was privy to an enormous range of secrets of joint operations that Britain and America were running against the Soviet Union.

In October, 1949, MI6 and the CIA send a team into Albania to try and topple the Communist regime.

The Albanian Army is waiting for them.

Kim Philby was able to tell the Soviet Union these operations were going on and it meant that the people who were dropped were effectively rounded up and, in some cases, executed.

You only have to have one person like that on the inside betraying you and a huge amount of what you do and your colleagues do is simply undermined.

So, you know, it's extremely damaging and a high vulnerability point for a service engaged in this kind of work.

Philby meets his KGB contacts in the exclusive and rarefied London suburb of Kensington.

This area of west London is just a mile and a half from the Soviet Embassy.

The Russians are confident their meetings with the traitor Philby won't be discovered.

They're running rings around MI6.

Around here, one of the most famous spy sites is Cafe Daquise.

It's a restaurant where the Cambridge spies are thought to have come to eat and to drink.

Near to the cafe is Holy Trinity Church, another location that the KGB use for their dirty work.

So this statue of St Francis of Assisi was used as a dead letter drop by the KGB during the Cold War.

It's a place where a message would have been hidden round the back of the statue which could then have been picked up by a KGB officer, so the two people would never have to meet, but information could be exchanged containing secret intelligence.

Another famous dead letter drop was at Mount Street Park in Westminster, yards from the US embassy.

A KGB agent would draw a chalk mark on a nearby lamppost to indicate he was leaving a message in the park.

A second KGB officer would remove the mark and head to the park to collect the message.

The KGB is far more adept, it's far more cunning, in some ways.

It's far better at putting agents inside Britain.

Britain doesn't have any agents inside the KGB.

It's not able to listen to their communications in the same way, so it's effectively blind and deaf against its enemy.

And that's a real disadvantage.

By the 1950s, Kim Philby comes under suspicion.

Questions are even asked in the Houses of Parliament about his loyalty.

He holds a press conference in which he publicly denies being a traitor.

It was a very capable performance by one of the great liars in British Intelligence.

Philby protests his innocence for another ten years.

But by 1965, the weight of evidence against him is too great and he finally flees to the Soviet Union.

Instead of receiving the hero's welcome he expects for all his hard years of service to the KGB, he is met with suspicion.

Stalin actually thought that they probably might not be what they seemed.

He actually thought that the Cambridge spies were so good they couldn't be true.

In Moscow, Philby finds himself under virtual house arrest.

The KGB won't employ him.

The disappointment is too much and he turns to drink.

In 1988, he dies a broken man.

Philby's betrayal is an enormous blow to MI6's relationship with the United States because Philby had enjoyed such an important position as the MI6 liaison with the CIA with Washington that suddenly America becomes distrustful of MI6.

It thinks to itself, 'Hang on a sec, how many more traitors might there be?

Should we be sharing as much information with MI6 as we were in the past?

Are these guys really as good as we thought they were?'

The British government cover up the entire Philby fiasco.

As far as the public are concerned, MI6 and its spies are still the brilliantly heroic characters of the James Bond stories - tuxedoed men armed with guns and clever gadgets.

The glamorous exploits might be fictional but the spy gadgets are a reality.

MI6 has a whole department that's dedicated to producing ingenious gadgets, in many cases.

For example I was given a briefcase that if I flicked the switches one way, it would turn on a concealed recorder.

But the ingenuity of MI6 is nothing compared to the deadly gadgets of the KGB.

On 7th Sept, 1978, London witnesses the use of a lethal device in a plot that is stranger than fiction.

Georgi Markov was an ambitious writer in Bulgaria who became dissatisfied with the Bulgarian regime in spite of having been at its topmost level.

He went hunting with the Communist dictator Todor Zhivkov, but defected and came to London.

MARKOV: ..in line with the corrupt nature of the Party and the regime... Markov was riling the Communist leadership in Bulgaria with his deeply critical writings and broadcasts.

MARKOV: We have seen how personality vanishes... For the Bulgarian regime, Georgi Markov was probably their number one public enemy.

It's just another routine day for Markov as he makes his way to work across Waterloo Bridge in London.

But within a few hours, he begins to fell unwell.

The following day, he is admitted to hospital suffering massive organ failure.

His doctors have no explanation.

Markov, though, is certain he has been poisoned.

I went in and saw him.

His words were, 'They're out to get me.

I've been poisoned by the KGB, there's nothing you can do about it, I'm going to die.'

Dr Riley is chief medical officer on call at the time.

His colleagues think Markov is paranoid and crazy.

Only Dr Riley believes his fantastic claims.

He just was so convincing and he seemed so genuine that I thought I'd better take this seriously.

Markov desperately tries to work out how he's been poisoned.

The only strange thing he can remember happening is at a bus stop on the way to work that morning.

He just told me about how he was waiting at a bus stop at the south end of Waterloo Bridge.

And he said that he suddenly felt like a sharp, stabbing feeling in the back of his right thigh.

And he turned around, to see someone there who'd accidentally prodded him with an umbrella.

Markov's condition deteriorates quickly and within three days, he is dead.

For the first time, MI6 fear the worst.

Has a foreign agent carried out an assassination on British soil?

Had someone been given a real-life license to kill?

Everything you're doing overseas as an operative is illegal, because espionage is illegal.

So the concept of having a license to kill is irrelevant.

If you kill somebody, you can't suddenly pull out a license and that's gonna be fine.

So the reality is, there's no need to have a license to kill.

License or not, Markov believes he's been targeted.

And whoever has attacked him has left a trace.

When I examined him and found on the back of his thigh sort of this puncture wound, I thought, 'Well, perhaps he has been poisoned.'

Buried inside Markov's leg, the pathologist discovers a tiny round pellet the size of a pinhead.

The microscopic holes drilled into it are so small, there's only one possible toxin powerful enough to have killed in such a miniscule dose.

Ricin.

You can imagine this is the pellet.

The theory is that the holes were sealed with something like gelatin which would melt at body temperature.

So that once the pellet is beneath the skin, the gelatin dissolves and the toxin leaks out.

The pellet was fired into Markov's leg by an elaborate mechanism most probably hidden inside the umbrella he saw dropped at the bus stop.

You need a lot of knowledge, a lot of skill and a lot of expertise to engineer something like the pellet containing the ricin that killed Mr. Markov.

MI6 believe the sophisticated nature of the pellet and umbrella device point to a state-ordered assassination - probably the work of the Bulgarian Secret Service with help from the KGB.

Our own Secret Intelligence Service must have been extremely embarrassed as well as concerned that this sort of attack had been carried out on Waterloo Bridge in broad daylight in the centre of London.

It was an incredible affront.

The Markov murder hits the headlines.

Suddenly MI6 finds its business splashed across the newspapers.

Its secrets are being laid bare in full public view.

By the late 1970s, it seems Britain's Secret Service isn't very secret anymore.

It was an open secret where the offices of MI6 were based at Century house just over the river in Lambeth.

It was supposed to be secret, there was no plaque outside saying MI6, but all the bus conductors in Lambeth used to say, 'Spies alight here, spies get off the bus here,' as a joke because everyone knew where MI6 was based.

After the disasters of the early Cold War, MI6's luck finally changes.

They persuade senior KGB member, Oleg Gordievsky, to turn double agent and leak Russian secrets to the British.

Gordievsky quickly climbs the KGB career ladder and in 1982, he's posted to a prime position at the Soviet Embassy in London.

Oleg Gordievsky was recruited by the British at an important moment when Britain and America did not have many spies who could reveal the inner thoughts, inner workings of the KGB leadership and of the leadership of the Soviet Union as a whole.

The MI6 officer in charge of handling Gordievsky is none other than John Scarlett.

He was turned into a really reliable and effective and committed agent, if you like, or double agent.

You can only do it if you trust the double agent concerned and by definition, trusting double agents is not a straightforward business.

But the British become convinced that Gordievsky is genuine.

During lunch breaks, he slips out of the Soviet Embassy to meet with Scarlett.

Very occasionally, he is also able to smuggle out top-secret documents.

Gordievsky is risking his life meeting Scarlett.

If the KGB catch a traitor, there is no reprieve.

They will execute him.

Gordievsky's most important moment is to come in late 1983.

Unknown to the public, for a brief moment, the world is on the brink of nuclear war.

Tension was increasing between the West and the Soviet Union when Ronald Reagan was talking about the evil empire and when the Soviet Union feared there could be a surprise attack.

NATO forces are preparing for a Europe-wide exercise that simulates a nuclear attack.

Everyone from the Joint Chiefs of Staff to the President is to be involved.

But the Soviet Union thinks the exercise is merely cover for the West to launch a real strike against them.

The world is unwittingly heading towards nuclear war.

Messages went out to the KGB station saying, 'You must be on high alert for the possibility of the West launching a strike against us.'

And the Soviet Union actually raised its state of alert in readiness.

In London, Gordievsky sees the aggressive messages from the Russian leadership and immediately warns MI6.

To pacify Moscow, the US immediately scale down the exercise and remove any involvement by the President.

The West suddenly realized just how edgy the situation was and just how volatile and dangerous things were.

That really helped London and Washington think much more carefully about how they managed that period of quite high tension during the latter part of the Cold War.

Gordievsky has proved his worth but in 1985, his luck runs out.

Though free to return to his Moscow home, Gordievsky knows his days are numbered.

He is closely watched at every turn.

In desperation, he asks MI6 to smuggle him out of the country.

His request goes straight to Number 10 Downing street where Prime Minister Margaret Thatcher authorizes MI6 to engineer a daring escape plan.

The rescue date is set.

On 20th July, 1985, Gordievsky waits at a pre-arranged spot near the Soviet-Finnish border.

At precisely 2pm, a car pulls up on the isolated road.

It is the British Secret Service.

Inside are a man and two women with a child.

MI6 is hoping a family outing will provide the perfect cover for the escape.

With Gordievsky in the trunk, the car speeds towards the border.

The Soviet frontier is heavily guarded.

(Dogs barking) And Oleg can hear search dogs outside.

If the dogs sniff him out, he's done for.

The strong smell from the diapers distracts the dogs and Gordievsky remains undetected.

Finally, after two tense hours, he hears the signal he has been waiting for.

(Radio switches to classical music) The car has reached neutral Finland and safety.

MI6 has repaid Gordievsky's loyalty and ensured his freedom.

Oleg Antonovich Gordievsky is a well-known public case and it is not possible to overstate the contribution he made to our national wellbeing and interests.

When the Soviet Union crumbles after the fall of the Berlin Wall, it seems like Britain and the West's game of cat and mouse with Russia is over.

But only 20 years later, the Russians are back and playing the spying game with a vengeance.

On 1st November, 2006, dissident Alexander Litvinenko is fatally poisoned at the Millennium Hotel in London, when his tea is laced with Polonium-210, a radioactive substance.

It's become clear that some of the old threats and some of the old dangers remain.

You've seen the resurgence of some of the concerns about Russian Intelligence operating in London, about Russia recruiting spies, even, it's thought, the Russian state killing dissidents on the streets of London in the case of Alexander Litvinenko.

The murder of Litvinenko wasn't just the KGB executing a troublesome dissident.

It was the start of an entirely new level of violence in the world of espionage.

Information came out showing that Alexander Litvinenko had actually been a paid agent of MI6, and he'd then been assassinated, it's thought, on the orders of the Russian state, by the Russian Intelligence Services, using radioactive polonium.

So, in a way, you can see, though the Cold War is gone, the spy games between Britain and Russia are still going on today.

The Russians have broken an unwritten rule in the spy game.

They have killed another country's spy, an MI6 agent.

The KGB may be no more but the new Russian Intelligence Service is just as deadly and operating on the streets of London.

Today, MI6 finds itself with a wider mission: preventing cyber attacks on Government organizations, stopping the spread of nuclear, chemical and biological weapons, as well as fighting terrorism.

It gradually became clear there was a new problem out there from international terrorism, not just a theoretical problem, actually a physical threat.

At least 20 foreign states are known to be actively targeting the United Kingdom.

MI6 is needed now more than ever.

My entire experience up until the present day is we face situations where we need to be well informed.

Self-evidently, the best form of defense against plots and attacks is to know about it in advance, and that is a matter of good intelligence.

As long as there are people who want to do Britain harm, Britain will need a secret service.

As long as there are threats out there, whether those threats are now from cyberspace and terrorists rather than the Soviet Union, then we'll need spies to try and understand those threats, to get inside them, to understand what your enemy's intentions are.

Because the United Kingdom is such a high priority target, the British Intelligence and Security Services have a budget in excess of $3 billion to fight hostile forces.

I largely suspect there are far more spies in London and in Europe now than there were during the Cold War.

London will always be the perfect place for spies to hide and pass unseen among the millions of visitors.

Of course, I would say this, wouldn't I?

But I genuinely, honestly cannot envisage a circumstance where our country will not need an effective intelligence service.

I've seen the importance of the service and the role it plays increase, rather than the other way around.

The service has continued to work, day in, day out since 1st October, 1909.

One operation leads to another operation, leads to another operation, that's how it is.

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