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. 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.