LONDON: Secret files have at last revealed the identity of the top spy who transferred Britain’s atomic bomb secrets to the Soviet Union and paved the way for the nuclear standoff with the west, triggering the Cold War for nearly five decades.

Though the MI5 suspected him, trailed him and monitored his every move, they were never able to get the man, codenamed “Eric” by the KGB, whose espionage campaign to steal the Allies nuclear bomb plans was codenamed Enormous.

Declassified MI5 files have confirmed that the master spy, described as the “main source”, was a Soviet mole at the Cavendish Laboratories at the University of Cambridge, the heart of the wartime nuclear research programme.

Today, 70 years later, with the opening of MI5 and KGB files, “Eric” can finally be identified as Engelbert (Bertie) Broda, whose story is a tale of espionage and counter-espionage, elaborate spycraft, love and deception.

Broda was the KGBs prize spy, who fed Britain’s nuclear secrets to Moscow for a decade, including the blueprint for the early nuclear reactor used in the US Manhattan Project, Times online reported on Thursday.

“Erics” secrets enabled the communist state to catch up in the race to build the nuclear bomb and set the stage for nearly five decades of nuclear standoff with the West.

Though the KGB archives of the period are now sealed, a brief window in the mid-1990s provided a KGB officer named Alexander Vassiliev access to the files.

Vassilievs notes form the basis of a new book, published in the US this month, revealing Brodas pivotal role in Soviet atomic espionage.

“Soviet sources in England were the first to provide Moscow with atomic intelligence,” wrote Pavel Fitin, Moscow’s head of Foreign Intelligence (1939 to 1946), in a memo quoted in Spies by Harvey Klehr, John Earl Haynes and Alexander Vassiliev.

According to Fitin, intelligence from Broda and others laid the groundwork for Soviet nuclear scientists, paving the way for the nuclear confrontation of the Cold War.

“The material included valuable and top-secret documents [that] served as a starting point for laying down the groundwork and organising work on the problem of atomic energy in our country,” the memo stated.

Among Brodas information included the blueprint for one of the American Manhattan Projects early nuclear reactors.

Broda, who was being heavily trailed by the security service (MI5), went back to Austria to teach at the University of Vienna in 1948.

Brodas son Paul, who remained with his mother in Britain, is writing a book about his father and stepfather, the British report said.

The most remarkable thing about the scientist-spy was his ability to evade detection.

In 1983, at the age of 73, the celebrated professor was buried in a “grave of honour”. Alongside that epitaph might stand another: “Eric”, the spy who got away.