Anthony Blunt - English gentleman, art adviser to Queen Elizabeth II and Soviet spy - felt the decision to give British secrets to the Kremlin was "the biggest mistake of my life."
Blunt wrote of his remorse in a 30,000-word memoir completed shortly before his death in 1983 and released Thursday by the British Library. It was given to the library in 1984 on condition it not be made public for 25 years.
Blunt was the infamous "fourth man" in a ring of upper-class Britons who spied for the Soviet Union. He, Guy Burgess, Kim Philby and Donald Maclean attended Cambridge University in the 1930s and - fired by opposition to the spread of fascism across Europe - were drawn into espionage.
The document reveals few new details of Blunt's career as a spy, but recounts how he was recruited at Cambridge by the charismatic Burgess.
Blunt said he considered joining the Communist Party like many of his fellow academics, but was urged not to by Burgess. Burgess had already been recruited by the Soviets and told to "go underground" and get a job in the British government.
Blunt said that "Guy, who was an extraordinarily persuasive person, convinced me that I could do more good by joining him in his work."
"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," he added.
During World War II, Blunt worked for the MI5 intelligence agency - and handed over secret documents to the Soviets. The memoir gives few details about Blunt's espionage, and does not reveal the names of his Russian contacts.
Blunt wrote that after the war he tried to put spying behind him, resuming his career as an art historian and becoming Surveyor of the King's Pictures, a job he held under King George VI and his daughter, Queen Elizabeth II.
"In fact I was disillusioned about Marxism as well as about Russia. What I personally hoped to do was to hear no more of my Russian friends, to return to my normal academic life," he wrote.
"Of course it was not as simple as that, because there remained the fact that I knew of the continuing activities of Guy, Donald, and Kim."
Burgess and Maclean, both British diplomats, fled to Moscow in 1951 after Philby tipped them off that Maclean was about to be exposed. Blunt said a Russian contact advised him to defect, too, but he decided against it.
"I realized quite clearly that I would take any risk in this country, rather than go to Russia," he wrote.
Blunt and Philby both came under suspicion, but avoided exposure as Soviet spies. Blunt wrote that he was able to use his contacts in MI5 to get into Burgess's apartment and remove documents linking him and Philby to the case.
Philby continued to work for Britain's MI6 overseas intelligence service until he, too, fled to Moscow in 1963. Soon after, Blunt was denounced by Michael Straight, an American whom he'd helped to recruit three decades earlier.
He confessed to British spymasters, but in return for information was allowed to escape disgrace, keep his knighthood and continue as art adviser to the queen.
Blunt wrote that he believed, "naively," that the story would never be made public. However, in 1979 Prime Minister Margaret Thatcher publicly unmasked him as a Soviet spy, and he was stripped of his knighthood.
Blunt said he considered suicide but decided it would be cowardly to leave his friends and family with "the double shock of my suicide and the revelations which would have followed immediately."
Blunt said that after he was exposed he took refuge in "whisky and concentrated work."
The manuscript, which has been known about for years but never made public, is likely to disappoint historians because it does not contain any sensational revelations.
Art critic Brian Sewell, who knew Blunt, said the document was "a damp squib." He said Blunt's infamy prevented him from being able to research a more detailed version of events.
"He couldn't get to the newspaper libraries. Nobody would give him any help," Sewell said.
"In the end he simply gave up, so the broader picture never got anywhere near being written."