Operation Enormoz was a WW2 Soviet venture to recruit British and American spies who sympathised with communism. Although the Soviet Union had joined forces with the U.S and Britain against Nazi Germany, they began a widespread effort to gather Anglo-American bomb intelligence.
Melita Norwood nee Sirnis, British civil servant and committed communist, aligned herself with the Soviet Union becoming part of the Woolrich Spy Ring in the mid-1930s. She was employed as secretary to the director of the Tube Alloys Project at the British Non-Ferrous Metals Research Association and passed nuclear secrets to the Soviets well into the 1970s. Norwood, along with other communist spies, enabled the Soviet Union to detonate their first nuclear weapon. She was formally honoured by the Soviets in 1979 and exposed in Britain as a spy in the 1990s. She is known as the most important and longest-serving KGB agent in Britain. More on her here.
Although Britain’s feminine cultural shadow of the time allowed Norwood to operate undetected, her long espionage tenure would have been interrupted earlier if the U.S./U.K. decryption project, Verona (to decipher Soviet telegraphs) had not remained secret into the 1990s. The secret classification also made it impossible to prosecute her and others.
For more information about the widespread KGB manipulation in the west and Europe, see The Mitrokhin Archive: The K.G.B. in Europe and the West.