Britain's spy chiefs continued actively to protest the innocence of Kim Philby for years after he helped fellow Soviet agents Guy Burgess and Donald Maclean to flee to Russia, according to a newly released Downing Street file.
The head of the Secret Intelligence Service (known as MI6) said it would be "un-English" to brand Philby a traitor on suspicion alone, pointing out that the master-spy had helped unmask other Soviet agents, including Maclean.
"It is entirely contrary to the English tradition for a man to have to prove his innocence even when the prosecution is in possession of hard facts," argued Major-General Sir John Sinclair, chief of the SIS in the mid-1950s.
"In a case where the prosecution has nothing but suspicion to go upon, there is even less reason for him, even if he were able to do so, to prove his innocence."
The blind faith placed in Philby by his old service, despite compelling evidence of his treachery, is disclosed in documents prepared in 1963 for Harold Macmillan, the then prime minister, after Philby's defection to the Soviet Union from Beirut in January of that year.
Maj Gen Sinclair's defence, dating from 1955, is contained in the previously top secret file.
In 1951, Philby had been forced to resign from his position as the senior SIS liaison officer in Washington following the disappearance of his fellow Cambridge spies, but there was insufficient evidence to bring him to trial.
Suspicion lingered, however, and in 1955 fresh evidence implicating him as the so-called 'Third Man' surfaced in Australia from a Soviet defector. A question was duly tabled in parliament naming Philby as the Third Man.
Helenus Milmo, a lawyer acting for the security service, MI5, who led the 1951 investigation of Philby, concluded: "I find myself unable to avoid the conclusion that Philby is and has for many years been a Soviet agent."
But four years later, MI6 was still defending its man. Writing to Mr Macmillan, who was then foreign secretary in the government of Anthony Eden, Maj Gen Sinclair pointed to evidence that "reduced very considerably the suspicion that Philby was a Soviet agent".
Philby had not only provided information leading to the exposure of Maclean, a senior British diplomat in Washington, but had also helped unmask Klaus Fuchs and Allan Nunn May, scientists instrumental in betraying the secret of the atomic bomb to Moscow. (© Daily Telegraph, London)