A British journalist who worked for the Daily Express betrayed his country to become one of the key Soviet agents of the Second World War.
Cedric Belfrage leaked highly sensitive secrets to the Russians while working for the British security services in the US.
The information was of such value that for a time Soviet intelligence regarded him as one of its key assets, even more important than Kim Philby, a member of the "magnificent five" Cambridge spy ring.
Previously unseen MI5 files revealed Belfrage had passed on intelligence about British spying methods to the Russians, along with highly sensitive documents on Vichy France and details of British policy in the Middle East and Russia.
But when he was finally snared he claimed the information was of a "trifling nature" and maintained he was using the intelligence to try to infiltrate Soviet networks.
Professor Christopher Andrew, the former official historian of MI5, said Belfrage's Soviet handler praised the intelligence he provided to Moscow as "extremely valuable".
He added: "For a year or so in the middle of the Second World War, Soviet intelligence even rated him ahead of Philby.
"Though Moscow has released some of Philby's KGB file, however, it has revealed nothing about Belfrage."
Born in London in 1904, Belfrage studied at Cambridge University, where he developed a passion for film, but left without taking a degree.
By the beginning of the 1930s he was Britain's best paid film critic, working as a film and theatre critic for the Daily and Sunday Express newspapers from 1931 to 1934, and as a critic and columnist for the Daily Express from 1935 to 1936, before moving to Los Angeles.
But he also had left-wing sympathies, and after visiting Moscow in 1936 he returned to the US a committed Communist, keeping close contact with Earl Browder, head of the Communist Party in America, as well as Communists in Britain and writing for numerous left-wing publications.
From 1941 to 1943 Belfrage worked for the combined MI6 and the Special Operations Executive body, the British Security Co-ordination, in New York – a letter in the files from Philby confirms his position – where he was the right-hand man of William Stephenson, head of the BSC and the most senior British intelligence officer in the western hemisphere.
This work took him on to the psychological warfare division of SHAEF, the Supreme Headquarters Allied Expeditionary Force in Paris, where he dealt with press affairs.
But he was exposed after the spy Elizabeth Bentley defected from the Communist Party to become an informant for the US.
In 1946 evidence from an FBI investigation into the Soviet secret police operative Jacob Golos and a defector codenamed "Speed" uncovered Belfrage's deception.
Authorities learned he had been passing intelligence to the Soviets during the 1940s and was part of a spy network engaging in espionage.
Files released by the National Archives at Kew, west London, reveal he supplied Golos with a report from Scotland Yard on methods of training intelligence agents.
It also contained notes from "some prominent burglars in England" on "surreptitiously opening safes, doors, locks, and other protective devices".
Belfrage handed over intelligence on British policy in the Middle East and Russia, as well as information from high-ranking British officials in the US.
When questioned by the FBI in June 1947, he confessed to leaking secrets to the Soviets, some via leading communist VJ Jerome.
He said: "My thought was to tell him certain things of a really trifling nature from the point of view of British and American interests, hoping in this way to get from him some more valuable information from the Communist side.
"I supplied him with information about Scotland Yard surveillances and also with some documents relative to the Vichy Government in France, which were of a highly confidential nature with respect to their origin but which contained information of no value whatever."
Belfrage gave evidence before the Federal Grand Jury, but it was decided he had not broken the US Espionage Statute as the documents leaked were British, not from the US government, and so he never stood trial.
Britain intelligence still wanted to get its hands on him, with a note from MI6 in August 1950 stating "should he ever be unwise enough to return here we should have an account to settle with him".
In May 1953 he was arrested and brought before Senator Joseph McCarthy's Senate Investigation Committee – where he refused to answer questions on whether he was a Communist.
The following month MI6 said it was still keen to prosecute him under the Official Secrets Act.
But it conceded that because Belfrage claimed he only leaked information to gain more valuable intelligence in exchange, and as he was unaware that what he told his Communist friends was reaching the Russian intelligence network, "it might be difficult for the Security Service to produce a strong enough case to warrant prosecution".
MI5 agreed, saying "at the moment there is no such evidence".
Belfrage was eventually deported from the US in 1955, and while he remained of interest to the security services in Britain he was not suspected of any further espionage. He died in Mexico in 1990, aged 85.