In the dim light of the Wormwood Scrubs arc lamps, George Blake could see a rope ladder sailing over the wall towards him.

The slender rungs writhed for a brief moment before the ladder settled and hung motionless.

Handmade from knitting-needles and lengths of nylon clothes line, it looked fragile but, as Blake himself would later say: 'The moment I saw it I knew nothing now would stop me.'

A prolific Cold War traitor, Blake had been sentenced to 42 years in prison, the longest term in modern British history, with no prospect of early release.

ESCAPE: Blake in the 1960s. A prolific Cold War traitor, Blake had been sentenced to 42 years in prison, the longest term in modern British history, with no prospect of early release

But now, just five years after entering 'The Scrubs', he was making an infamous escape.

On October 22, 1966, as his fellow prisoners finished watching a film, Blake slipped out of the main building through a carefully prepared hole in a window, climbed down a drainpipe and concealed himself in the darkness of a recess.

Then, with no patrols in sight, he grabbed the flimsy ladder and scrambled up the perimeter wall to freedom.

George Blake spent a decade passing Western secrets to the KGB, a torrent of information including, unforgivably, the names of men and women working for British intelligence behind the Iron Curtain.

Codenamed Agent Diomid, the Soviets regarded Blake as so valuable that even the head of the KGB in London, the 'rezident', knew nothing of his activities.

I have spent five years sifting through previously unseen documents in British, American and German archives, conducting interviews with dozens of key figures, including Blake himself.

And what emerges is not just the enormous scale of his espionage and the depth of his commitment, but the extraordinary damage he did to Britain and the West.

Blake was particularly well-suited to spying.

The son of a Dutch woman and a Turkish citizen who had fought for the British Empire, he had a cosmopolitan background, experience in hazardous situations and a facility for languages, including Russian, German, French, and Dutch.

After the Nazi German conquest of the Netherlands in 1940, Blake, still a schoolboy there, served heroically, delivering messages for the resistance.

In 1942, he made a daring escape from occupied Europe through France and Spain before reaching England, where he joined the Royal Navy – and where his potential was spotted by the Secret Intelligence Services (SIS), also known as MI6.

In 1949, Blake was posted to Korea, his mission to spy on the Soviets in the Far East.

The following year, however, he was captured by the Russian-backed North Korean army which invaded the south and engulfed the peninsula in war.

And it was during his three years in captivity that he took a monumental decision: for reasons he would always describe as purely ideological, Blake offered his services to the Soviet Union. The effects would prove to be devastating.

Blake's career of treachery began in earnest in London on September 1, 1953, in a Georgian mansion at 2 Carlton Gardens near St James's Park.

He had been assigned to work in a special section of the Secret Intelligence Service, established to exploit the potential of penetrating the Soviet Union by tapping its telephone landlines.

CAREER OF TREACHERY: George Blake sitting in the kitchen of his dacha

The project needed a deputy who spoke good Russian and Blake fitted the bill. His KGB controllers must have been delighted, as the role put him at the heart of many top-secret operations then underway in Europe.

Blake's modus operandi was surprisingly straightforward: he photographed what documents he could, then passed them to his Russian handler at rendezvous near North London Tube stations or on the top deck of double decker buses.

He had been given a small Minox camera to snap photos of any interesting documents that landed on his desk – and many did.

He waited until the secretaries next door to his office were at lunch, or he would stay late, leaving the door of his office open so he could hear at once if anyone entered the outer room.

Occasionally, with larger reports, he found it easier to simply take away the whole document. The elderly watchman at the door never checked anyone's briefcase.

Now and then he saw his handler for quick 'brush passes', wordlessly handing over undeveloped film or documents as they passed on narrow streets. Every three or four weeks, they would meet to talk.

Blake was so prolific that he helped bring about what the Russians described as the 'complete elimination' of the Western spy networks in East Germany during the years 1953 to 1955.

In April 1955, he arrived in Berlin for a new SIS posting, ahead of his British wife Gillian who had no idea she was married to a Soviet spy.

His new official assignment was also highly sensitive: to penetrate the KGB headquarters there.

Blake was given the job of making contact with Russians in East Berlin, Soviet intelligence officers in particular, with the goal of recruiting them as agents. But his real work was for the Soviets, and it was not just documents that George Blake was turning over to the KGB.

It was names as well – the identities of agents working for SIS. Most of them were East Germans, though they included Soviets and other nationalities. As with the documents, Blake could not put a number on how many agents he betrayed.

'I can't say, but it must have been maybe 500, 600,' he later said.

By 1960, Blake had had enough. Hoping to leave espionage behind, he gained a posting to Lebanon to learn Arabic – but it was too late.

After years of treachery, he had himself been exposed by a Polish defector. Lured back to London in 1961, Blake was arrested and, during his interrogation, confessed.

His wife, Gillian, was horrified, she said, at 'the unbelievable news that this man, to whom I have been so happily married for nearly seven years, was in prison awaiting trial as a Russian spy.'

Pleading guilty to five charges of breaking the Official Secrets Act at the Old Bailey, Blake had expected to receive a maximum sentence of 14 years, but Lord Parker, the Lord Chief Justice of England and Wales, had an unpleasant surprise in store.

'Your conduct in many other countries would undoubtedly carry the death penalty,' he said.

'In our law, however, I have no option but to sentence you to imprisonment, and for your traitorous conduct extending over so many years there must be a very heavy sentence.'

Parker imposed a sentence of 14 years for each count, three of them to run consecutively and two of them concurrently, 42 years in all. There were audible gasps in the courtroom.

Escape came naturally to Blake. During his 1,000-mile journey across Europe in the war, he had jumped off a moving train in Belgium to evade German soldiers and trekked across the snowy Pyrenees on a mule trail.

By 1965, he had finally realised that the KGB were not going to get him out of the dingy Victorian confines of Wormwood Scrubs.

If he was going to be freed, he needed people who were still on the inside but due to be released soon, who could co-ordinate a plan.

He settled on Sean Aloyisious Bourke, an Irishman with an anti-authoritarian bent who was serving an eight-year sentence for attempting to kill a police officer.

He also enlisted the help of Pat Pottle and Michael Randle, two idealistic anti-nuclear campaigners he'd met in the prison's music appreciation class.

Bourke planned to make a rope ladder to scale the 20-foot wall, but wooden rungs would make it heavy and noisy, so Randle's wife Anne proposed making the rungs with knitting needles.

A size 13 needle, made of steel and coated in plastic, would be strong enough to hold a man yet add little weight, she suggested.

Bourke bought nylon clothes line at Woolworths, and then went to a nearby shop to buy the needles. 'I'll have 30,' he told the sales clerk.

Incredulous, she raised her eyebrows and said: 'Your wife must be doing a lot of knitting.' Bourke made last-minute preparations, buying a second-hand television he could use to monitor the news and some clothes that Blake could wear.

The escape was set for the evening of Saturday, October 22, when the other inmates would be watching a film and well away from D Hall, the block housing Blake.

A few days in advance, another prisoner – an expert at breaking and entering – had loosened two panes of glass from the tall Gothic-style window that took up much of the exterior wall and Blake now had possession of a walkie-talkie, smuggled in for him so that he could talk to the newly free Bourke.

As darkness fell, Blake slipped out through the window, made his way to the edge of the roof and dropped to the ground, telling Bourke over the radio that he was ready for the ladder.

But Bourke sounded agitated.

'Hang on just a sec,' he replied. 'I am up against an unexpected snag.' A security guard in a van was driving down the road and evidently thought there was something suspicious about a car parked so near the prison wall – Bourke's.

He stopped his van and stepped out, holding a large Alsatian guard dog on a leash. He stared in at Bourke, making no attempt to hide his suspicions.

Spooked, Bourke drove off. But after driving a circuit, he returned to find the van gone and finally radioed Blake.

Bourke could hear great anxiety in Blake's voice. 'Well, I'm already out of the hall and waiting for that ladder! The men are back from the cinema. The patrol might come along any moment. Hurry!'

Bourke pulled the ladder out of the car boot. He climbed on to the roof of his car, swung the folded ladder with his right arm three times and then tossed it over the wall.

Blake tumbled awkwardly and smashed his head on the gravel when he made it over the wall, but Bourke dragged him to the car, opened the rear door and pushed him into the back seat.

Rain was beating down on the car as they drove away. Bourke put his foot down.

Blake's forehead was bleeding, his left wrist was broken and bent at an ugly angle, but he was too elated to feel much pain.

A huge manhunt failed to find him. While the country's ports were watched and his photograph was displayed on television and the front pages, Blake was lying low in a nearby bedsit.

He was eventually smuggled across the Channel by Randle in a camper van, then across northern Europe and through West Germany to the border crossing. In East Germany, he identified himself to the border guards and completed his escape to Russia.

Arrving in Moscow in January 1967, Blake did not take long to recognise that communism in the Soviet Union was an utter failure. The deprivations of The Scrubs prepared him well for the endless shortages.

Two months after his arrival, he discovered Gillian had been granted a divorce in his absence.

Blake stayed at first in a spacious KGB apartment with a housekeeper to prepare his meals.

He spent his days writing long memos for the authorities, compiling information he had been unable to pass on before his arrest, as well as details of how the British had interrogated him.

Life improved markedly when, on a Volga riverboat cruise in the spring of 1968, he met Ida, a Russian woman who worked as a French translator for an economics institution.

The two settled down together in a spacious apartment organised by the KGB and Blake was given privileges and medical benefits similar to a military general.

After the birth of their son, Mischa, in 1971, he was presented with a dacha, a holiday home, in the countryside around Moscow.

For more than 30 years he worked at the Institute of World Economy and International Relations, a leading Moscow think-tank.

When the Berlin Wall came down and the Cold War ended, Blake seemed to think he would be forgiven back home. He was soon disabused.

In fact, according to Markus Wolfe, the head of the former East German spy agency, lifting the Iron Curtain served only to shrink Blake's world still further.

He was left to 'live out a withdrawn life in an adopted homeland that had abandoned his cause'.

Today, aged 96, his eyesight has faded to the point that he is nearly blind. Many former KGB officers have dachas in the area and neighbours are protective.

Blake's wooden house is on a corner plot behind a tall fence and is watched over by a friendly caretaker with a beret, and a small white schnauzer, Plushka.

Inside the four-bedroom dacha the decor is simple and it is kept cosy all year round by central heating.

Blake still ventures out for walks with Ida, 13 years his junior, hunched over and holding her arm.

Because his eyesight is so poor, he can no longer watch the BBC via satellite as he used to, so Ida reads to him.

In addition to Mischa, Blake occasionally sees his three English sons, who with Gillian's blessing began visiting their father in the 1980s and are now close to him.

Blake is an honoured figure in Russia, decorated with medals for his espionage, including the Order of Friendship medal in 2007 presented by his admirer, President Putin.

He is lionised by the Russian press, which calls him the 'grave digger of British intelligence', and carries his occasional pronouncements on the state of the world, usually made during annual birthday interviews with official organs.

'The American empire will disappear because everyone who lives by the sword dies from the sword,' he told Izvestia, the official government newspaper, in 2010.

Blake's 90th birthday, on November 11, 2012, was a big event, attended by all of his children.

In the afternoon came a congratulatory call from Putin, who saluted Blake for his 'enormous contribution to the preservation of peace', declaring that the spy had earned a place in the 'constellation of strong and courageous men'.

Yet the praise from Putin is a source of discomfort for Blake. He has told friends that he loathes the Russian president and the cynical and violent authoritarian rule he has imposed.

Today, in describing his espionage, Blake clings to the fanciful idea that none of the hundreds he betrayed ever came to serious harm. 'I challenge anybody . . . to name one who has been executed,' he wrote.

But according to KGB general Oleg Kalugin, who oversaw Blake in Moscow, he has an 'innocent mind'. Kalugin recalled that Blake told him several times that the KGB had promised that none of those he identified had been shot.

'He clung naively to that belief, and I didn't have the heart to tell him that his work led directly to the deaths of dozens of agents behind the Iron Curtain,' Kalugin wrote.

Thanks to the astonishing scale of George Blake's treachery, the Soviets received invaluable information about Western intelligence, its organisation and the people working for it.

And thanks to Blake, they knew from the very start about the audacious plan to build a clandestine Berlin spy tunnel right under the feet of our Cold War adversaries – a remarkable story to be recounted in next week's Mail on Sunday.

© Steve Vogel, 2019