In his autobiography, published last year, two years after his death, Cairncross admitted spying for the Russians but said he had only helped them in their fight against Hitler.
But KGB files, made available to Rupert Allason, the former Tory MP who is also known as the author Nigel West, demolished his claims. They show that he handed over British atomic secrets, betrayed the identities of British agents, and was well paid by the Russians for his spying.
The files contain a memorandum by Pavel Fitin, the KGB's head of intelligence, on Enormoz, the Soviet programme for obtaining Allied atomic secrets.
"The first material on Enormoz was received at the end of 1941 from John Cairncross," it notes. "This material contained valuable and highly secret documentation, both on the essence of the Enormoz problem and on the measures taken by the British government to organise and develop the work on atomic energy.
"This material formed the point of departure for building the basis of, and organising the work on, the problem of atomic energy in our country."
The revelation yesterday prompted Teddy Taylor, Tory MP, to table a parliamentary question for Jack Straw, the Home Secretary, asking for the "full facts". He said: "This is an issue of the utmost urgency and it raises the most alarming suspicions about what went on in our intelligence services at this critical period in our history."
Allason has co-authored his book, The Crown Jewels, with Oleg Tsarev, a former KGB officer who works as a consultant to the KGB's successor, the SVR. "I have had to rely on him for the Russian documents," Allason said yesterday.
But, he added, there was no secret Russian agenda. "They are proving that they're a democracy - that they've got declassification, and they are more open than we are."
Other historians were more sceptical. Donald Cameron Watt, an emeritus professor of international history at the London School of Economics, said: "This is an indication, not so much of what happened, as what the KGB wants us to believe happened."
Cairncross was born in 1913 near Glasgow; his father was an ironmonger. He won a scholarship to Trinity College, Cambridge, where he met Anthony Blunt, another member of the Cambridge ring.
The files show that Blunt introduced him to fellow Moscow sympathiser Guy Burgess, who made an assessment of the young Scot for the KGB. Cairncross was later recruited by Andre Deutsch, or Otto, the KGB officer running the Cambridge ring.
Cairncross, or agent "Liszt", supplied his most valuable information when he was secretary to Lord Hankey, minister without portfolio in the Churchill government, who had special responsibility for the intelligence services and for atomic research.