John Herbert King

John Herbert King, alias 'MAG', was a British Foreign Office cypher clerk who provided Foreign Office telegraphic traffic to the Soviet Intelligence Service between 1935 and 1937. He was sentenced to 10 years' imprisonment as a spy in October 1939.

King had been recruited into the Foreign Office as a temporary clerk in 1934 and sent on temporary duty to the British Delegation to the League of Nations in Geneva. There, having financial problems, he was identified as a potential spy by Henri Pieck, a Dutch citizen who was working for Soviet intelligence. Pieck recruited him, pretending the information he gave was for the commercial advantage of a Dutch bank. King returned to London in early 1935. Pieck continued to run the case by visits to London until 1936, when security concerns caused the Russians to transfer control of the case to Theodore Maly. King continued to pass copies of Foreign Office telegraphic traffic to Maly until June 1937, when Maly was recalled to Moscow. In September 1939 Walter Krivitsky exposed King's name as a spy for the Soviet Union to the British Embassy in Washington. Coincidentally a business associate of Pieck's in London reported suspicious activities by him and described a man like King who had given information to Pieck. King was subsequently interrogated, resulting in a confession.

Information passed on by King is credited with giving Stalin valuable insight into British diplomatic activities aimed at containing Hitler in 1939. At times this information was passed on by the German Embassy in London, with the aim of increasing the tension between Britain and Germany.