John Cairncross was among the most damaging spies of the twentieth century. A member of the infamous Cambridge spy ring, he leaked highly sensitive documents from Bletchley Park, MI6 and the Treasury to the Soviet Union - including the first Atomic secrets and raw decrypts from Enigma and Tunny that influenced the outcome of the Battle of Kursk. Based on newly released archival materials, this biography will be the first to cover the life and espionage of this singularly important spy. In 2014 Cairncross appeared as a secondary, though key, character in the biopic of Alan Turing's life, The Imitation Game. The result was considerable negative reaction within the national press. Despite this clear interest, the function filled by Cairncross remains an untold story. Where all of the other members of the Cambridge spy ring have been the subject of extensive biographical study, Cairncross has largely been omitted by both academic and popular writers, and no biography has yet been penned of his life.