Double-Cross System (XX)

A Second World War MI5 operation in which captured German spies were used as channels for passing on false information.

Description based on the Crypto Museum's crypto glossary.¹

According to MI5, the Double-Cross System grew out of work with Christopher Draper (the “Mad Major”) in the mid-1930s. From January 1941 the deception was coordinated by the Twenty Committee, chaired by the historian J. C. Masterman; the name comes from the Roman numeral for twenty, XX, which is a double cross. The first “turned” agent was SNOW, a Welsh electrical engineer spying for Germany; in all, MI5 took over 120 German agents during the war. Masterman wrote that as a result “we actively ran and controlled the German espionage system in this country”.²

The most famous of the agents, GARBO (Juan Pujol García), run by Tomás Harris, convinced the Germans that he had a network of 28 sub-agents and helped persuade them that the main Allied landing would come in the Calais area; Berlin awarded him the Iron Cross (Second Class). According to MI5, four weeks after the Normandy landings in June 1944 Churchill was told that some German divisions had been held back from Normandy because of the supposed threat to the Pas-de-Calais. MI5 stresses that the Bletchley Park codebreakers provided indispensable support for the system.²

More in the service card: MI5 (Security Service).