Joined the Tube Alloys Project, helping to perform research on the development of the atomic bomb in 1942. Sent to Canada to perform further atomic bomb research in Ottawa in 1944. Was approached by representatives of Soviet Colonel Nikolai Zabotin, a military attaché for the Soviet Embassy and an intelligence officer for the GRU.
Visited the Chicago-based atomic research center and met with Major-General Leslie Groves, the head of the Manhattan Project. In 1944. Returned several times to Chicago to conduct experiments with atomic piles and would meet several times with top scientists to discuss the design and development of an atomic bomb.
Provided information about the experimental test blasts in New Mexico and then delivered plutonium and uranium samples to Zabotin.”I provided all the information I could,” he would later state in a death-bed confession. “I would have stopped at this point except there were crucial developments in the nuclear project in the US and there was a lot of information about the American decision to use the nuclear bomb against the Japanese and the development of nuclear power in America.” In 1945, Igor Gouzenko, a cipher clerk in the Soviet Embassy in Ottawa, defected to the Canadian government. Gouzenko helped to expose spy rings in the United States and Canada and May was placed under surveillance by MI5.
Returned to England in 1946, having arranged with Zabotin to meet with a new Soviet contact at the British Museum in London. Began lecturing in physics at King’s College.
Was interviewed by Lt. Col. Leonard Burt, a representative of Scotland Yard. Burt explained that the interview was simply routine, but then stunned May by informing him that MI5 was aware that he had failed to attend his meeting with the Soviet contact at the British Museum. May quickly confessed his espionage activities, explaining that he did so as a contribution to mankind (he had only received minimal payments from the Soviets).
Refused to provide any information about the spy ring, instead admitting that he had passed along information to the Russian, who were allies during the war, therefore shielding himself from a possible death sentence for collaborating with the enemy.
Plead guilty to treason on May 1, 1946 and was sentenced to ten years in prison at the Wakefield Prison in Yorkshire. He said that he pleaded guilty in order to conceal how long he had been a Soviet spy and the extent of the Russian networks in Britain. Was released in 1953 after time off for good behavior. Became a professor of physics at the University of Ghana and was later believed to have returned to the employ of the Soviet Union.