Anthony Blunt, one of Britain's most notorious Cold War spies, admitted that spying for Russia was "the biggest mistake of my life," in memoirs released on Thursday, 26 years after his death.
Blunt became known as the "Fourth Man" of a spy ring recruited among academics at Cambridge University, and went on to work for MI5 and leak hundreds of secrets to Moscow.
He wrote his memoirs after that, with the stipulation they should not published until a quarter of a century after his death, in 1983. Under those terms the 30,000-word manuscript was lodged in the British Library on Thursday.
Blunt, a don at Trinity College, met the flamboyantly homosexual student Guy Burgess in 1931, and was persuaded by him not to join the Communist Party, despite intense pressure from left-wing colleagues in Cambridge.
Instead, Burgess recruited him as a fellow spy for Soviet leader Joseph Stalin's Comintern.
"I might have joined the Communist Party, but Guy, who was an extraordinarily persuasive person, convinced me that I could do more good by joining him in his work," Blunt wrote.
"What I did not realise at the time is that I was so naive politically that I was not justified in committing myself to any political action of this kind," he said.
"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."
Blunt left Cambridge in 1936 and subsequently worked for the MI5 domestic intelligence service, passing hundreds of secret documents to his handlers in the NKVD, the forerunner of the KGB.
The other members of the Cambridge spy ring were Harold "Kim" Philby and Donald Maclean. Burgess and Maclean defected in 1951, followed by Philby in 1963.
In his memoirs Blunt says he had hoped to end his spying activities after World War II and resume his academic life as an art historian.
"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."
He was eventually exposed by then premier Margaret Thatcher in the House of Commons in November, 1979. The prospect of this almost led him to commit suicide, he wrote.
"Many people will say that it would have been the 'honourable' way out. After a great deal of thought I came to the conclusion that it would on the contrary be a cowardly solution," he said.
After being named he went into hiding, taking refuge in "whisky and concentrated work." He is thought to have lived somewhere in Europe, before returning to London, where he died.
In the memoirs Blunt insisted that there was "nothing sexual" in his relationship with Burgess, although both of them were gay. But he said he was won over by "the liveliness and penetrating quality of his mind".
"He could be perverse both in argument and in behaviour, but in the former he would wriggle back to sense and in the latter he would apologise in such an engaging manner that it was difficult to be angry for long," Blunt wrote.
"His sex life was already fairly full, but he did not blazon it about as he was to do later," he said.