On 17 October 1946, Joseph Stalin received a Russian translation of the top-secret document "Strategic Position of the British Empire". It wasn't the first time secret information from London became known to the leadership of the USSR, thanks to the activities of the "Cambridge Five" - a ring of Soviet spies in the UK, formed in the 1930s at the University of Cambridge by the Soviet spy Arnold Deutsch.
The "Cambridge Five" involved the most reliable spies operating in the UK, who were chosen by Soviet intelligence officer Yuri Modin: Kim Philby, Donald Maclean, Anthony Blunt, Guy Burgess, and John Cairncross. The leadership of the Soviet intelligence community was quite surprised by the desire of British intelligence agents and civil servants from elite circles to transmit secret information to the USSR. John Cairncross, for example, did not consider his work for the Soviet Union to be a betrayal, believing that a group of right-wing politicians in London were hiding the information necessary to defeat Nazi Germany from their ally, and subsequently sought East-West nuclear parity.
The "Cambridge Five" helped the Soviet Union to start work on the atomic project in 1941 and to prepare for the decisive battle of Kursk in 1943. In 1947, the members of the "Five" transferred the full package of documents on the creation of NATO - the financing scheme, composition, etc.
All five were high-ranking British intelligence or diplomatic officials. Philby, Maclean, and Burgess were subsequently transferred to the USSR, and Cairncross, after being discharged from the civil service, moved to Chicago and was only officially discovered in 1979. He spent a year in prison in Rome. Blunt stayed in the UK and wasn't prosecuted. According to CIA Director Allen Dulles, the "Cambridge Five" was the most powerful intelligence group during World War II.
Source:
John Fieher. Burgess and Maclean