Newly declassified British intelligence files have revealed the identity of a spy who passed military secrets to the Soviet Union during World War II.

The files, which were released on Friday by the British internal security service MI5, reveal that author and journalist Cedric Belfrage passed highly sensitive information to Russian operatives during World War II, when he was employed as a New York-based assistant to William Stephenson, the most senior British intelligence agent operating in the Western Hemisphere.

According to the files, among the documents Belfrage handed over to the Soviets were summaries of briefings with British Prime Minister Winston Churchill. Christopher Andrew, the former official historian of MI5, told the Financial Times that between 1942 and 1943, Soviet intelligence valued Belfrage more highly than his more famous counterpart, Kim Philby.

Belfrage attended Cambridge University, preceding the members of the so-called "Cambridge Five" spy ring (Phily, Donald Maclean, Guy Burgess, Anthony Blunt and John Cairncross) by almost a decade. After dropping out of university, he moved to America, and briefly joined the Communist Party in 1937.

Belfrage's purple patch as a Soviet spy came to an end in 1943 when his handler suddenly died. The handler's replacement was Elizabeth Bentley, who defected from the Communist Party in 1945 and handed the names of dozens of Soviet agents to U.S. officials.

Remarkably, when Belfrage was brought in for questioning by the FBI in 1947, he admitting passing documents to the Soviet Union during the war, but maintained that he had been working as a double agent. In Belfrage's telling, he had given the Russians meaningless information to bait them into providing him with vital data. 

FBI efforts to check Belfrage's story with MI6 went nowhere. Higher-ups in British intelligence responded that they could not hand over any "actionable" intelligence on Belfrage. In reality, the files seem to indicate that British intelligence feared the embarrassment that would result from the exposure of a known Communist sympathizer so close to the top of their hierarchy.

The year after his brush with the FBI, Belfrage helped found the far-left National Guardian newspaper, which brought him to the attention of the House Un-American Activities Committee. 

Belfrage was deported from the U.S. in 1955, and denied being a Soviet spy until his death in 1990 at the age of 85. Unlike three of his more famous fellow spies -- Maclean, Burgess, and Philby -- Belfrage never actually defected to the Soviet Union, and he was never prosecuted for his crimes. 

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