Upon the outbreak of World War II, May allowed his Communist Party membership to lapse and began working on the British atomic weapons program, known as the Tube Alloys Project. In 1943 he relocated to the Chalk River Laboratory near Montreal, Canada, that had become an annex of the U.S. Manhattan Project. That same year, Soviet military intelligence recruited him as a spy. Operating under the code name "Alek," May supplied his handler, Pavel Angelov, and controller, Colonel Nikolai Zabotin (Soviet military attaché in Ottawa), with a range of atomic secrets, including details about the Trinity and Hiroshima bombs, the output of plants, and microscopic samples of Uranium-235. Historical evidence now seems to suggest, however, that despite May's extensive and advanced knowledge of the atomic bomb, the information he passed on was of a general nature and therefore not particularly useful to the Soviets.
In May 1945 Igor Gouzenko, a Soviet Komitet Gosudarstvennoi Bezopasnosti (KGB) intelligence officer, defected to the West. During debriefings, Gouzenko revealed the nature of the atomic spy ring, directly implicating May. But because British intelligence hoped that May might offer further insight into Soviet penetration of the Allied atomic bomb program, he was permitted to return to King's College, London University. He was finally arrested in March 1946 and was charged under the British Official Secrets Act. He confessed and served six years of a ten-year prison sentence. Upon his release in December 1952, he was blacklisted. But in 1961 he was invited by President Kwame Nkrumah to work in Ghana. In 1978 May returned to Cambridge, where he died on 12 January 2003.
Hyde, H. Montgomery. The Atom Bomb Spies. London: Hamish Hamilton, 1980.; Moorehead, Alan. The Traitors. Rev. ed. New York: Harper and Row, 1968.