London, July 23 - A notorious British double agent who worked for the royal household and taught at Cambridge University, has said spying for the Soviet Union was the 'biggest mistake' of his life.

In a posthumous memoir opened for research by the British Library Thursday, Anthony Blunt -- an upper class, Establishment figure -- also said he chose to stay back in Britain rather than flee to the former Soviet Union when he was publicly denounced as a traitor and even contemplated ending his life.

Blunt, a Trinity College don and noted art historian, doubled up as a 'talent spotter' and member of the notorious World War II and 1950s 'Cambridge Spy Ring,' whose other members were Harold 'Kim' Philby, Donald McLean, Guy Burgess and John Cairncross.

Blunt, who was also the Surveyor responsible for maintaining the royal collection of paintings, admitted to being a Soviet spy in 1964 and was publicly unmasked by former prime minister Margaret Thatcher in 1979.

After his public exposure, he wrote a 30,000-word memoir, which was donated anonymously to the British Library in 1984 -- the year after his death -- on condition that it remained secret for 25 years.

In the memoir, Blunt, a wartime agent for the British spying agency MI5, said despite his enthusiasm for Soviet communism, he was persuaded not to join the British Communist Party by fellow spy Guy Burgess.

'I might have joined... but Guy, who was an extraordinarily persuasive person, convinced me that I could do more good by joining him in his work,' he 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.

'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.'

As war clouds gathered over Europe in the late 1930s, Blunt said he decided 'the ivory tower no longer provided adequate refuge'.

Both Blunt and Burgess were homosexual, but Blunt said they were not sexual partners.

Blunt said although he did not much care for undergraduate Burgess initially, he was later won over by 'the liveliness and penetrating quality of his mind' and interests.