Anthony Blunt was one of the royal family's most trusted employees. He was also a Soviet spy.

In 1979, a British solicitor made desperate attempts to stop the release of an upcoming book by a little known Scottish journalist. The Climate of Treason: Five Who Spied For Russia, told the sensational true story of a group of British elite at the centre of a Soviet spy-ring. Among them was the so-called ‘Fourth Man’, referred to in the book by the pseudonym ‘Maurice’.

That solicitor, it was later revealed, had been hired by a former employee of the Royal family: art historian Sir Anthony Blunt, Surveyor of The Queen’s Pictures.

This man, who’d lived and worked within the palace walls, who’d be a trusted Royal advisor for decades, was the true ‘Fourth Man’; a former KGB spy involved in one of the biggest security breaches of the 20th century.

Watch: The Crown S3 shines a light on some of the palace’s biggest secrets.

Sir Anthony’s extraordinary story is the hook of the opening episode of season three of The Crown, in which the academic is played by Samuel West. But the Netflix series only reveals part of the story…

How Anthony Blunt became part of the Cambridge 5 spy ring.

It was a friendship that led Anthony Blunt astray.

During his time at Cambridge University in the 1930s, he struck up a bond with undergraduate Guy Burgess, a flamboyant, charismatic young man, and enthusiastic Marxist. Like many students of the era, Blunt embraced extreme left-wing politics as a reaction to the rise of fascism and Hitler: he later wrote, “largely owing to the influence of Guy Burgess… I realised that one could no longer stand aside”.

It turned out that Burgess had been secretly recruited by another Trinity College student, Kim Philby, alongside Donald Maclean, to “go underground” for the Comintern, Stalin’s international Communist organisation. Blunt became the group’s ‘talent spotter’, and in turn recruited their ‘Fifth Man’, John Cairncross, and later an American named Michael Straight.

At their height, the so-called Cambridge Five infiltrated intelligence services, the Foreign Office, and the War Ministry, and passed some of Britain’s most sensitive secrets to the Soviet Union. Blunt himself worked for MI5 during WWII and handed over hundreds of documents, before starting a new career as an art historian that saw him appointed to the palace.

When Burgess and Maclean defected to Moscow in 1951 to avoid imminent exposure, other Cambridge academics — including Blunt — came under suspicion. But it wasn't until 1963, when Michael Straight volunteered information to US authorities about his involvement in the spy ring, that Blunt's role was confirmed.

The Queen learned the secret but kept quiet.

In exchange for a confession, Anthony Blunt was offered a secret deal that would grant him immunity from prosecution and keep him under wraps. He agreed and confessed to MI5 on April 23, 1964. The Queen was told shortly afterward.

Sir Anthony Blunt was allowed to carry on his privileged life as an art historian, and continued in his royal role until 1972; his colleagues and many within the palace remained blissfully ignorant about his past.

It wasn't until his attempts to demand a typescript of The Climate of Treason that the truth went public.

The unmasking of the fourth man.

Magazine Private Eye ran with the details of Blunt's attempts to quash the book, thereby publicly linking him to the Cambridge Five scandal.

Ten days after the book hit the shelves, Britain's new Prime Minister, Margaret Thatcher, told the House of Commons — and the world — what the Queen and British intelligence had kept secret for years: Anthony Blunt was the Fourth Man, a traitor.

Minutes later, Buckingham Palace announced Blunt would be stripped of his knighthood.

Four years later, in 1983, Anthony Blunt died of a heart attack.

Before his death, he penned his own version of the extraordinary events in a 30,000-word autobiographical manuscript; his history, through his eyes.

The text was bequeathed to a friend who, a year later, passed it to the British Library along with strict instructions that it was not to be released for another 25 years.

True to their word, the library made the document public in July 2009. Within it, Blunt seemed to express his guilt for being involved in what he called the "Russian nightmare", and the traitorous actions that would come to define his legacy.

"What I did not realise is that I was so naïve politically that I was not justified in committing myself to any political action of this kind," he wrote. "The atmosphere in Cambridge was so intense, the enthusiasm for any anti-fascist activity was so great, that I made the biggest mistake of my life.”

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