The Double Agent

These are agents who turn against the intelligence service that originally recruited them and work for another agency, while at the same time making the original agency believe their loyalty has not changed. They may do this for ideological reason, for personal gain, or to save their lives after being captured. Double agents are a dangerous threat to intelligence operations, as they can be used by their new controllers to feed misleading information to their original employers.

During World War II, Britain's MI5 set up an organization known as the Twenty, the XX, or the Double Cross Committee. This body clandestinely controlled much of Germany's intelligence-gathering effort in Britain. It went so far as to set up a series of largely fraudulent "German" intelligence networks that reported to the Germans without arousing suspicion, but were in reality under the control of MI5.

Two of the most successful double agents actually voluntereed their services to British intelligence during World War II. Dusan Popov was a Yugoslav who ran a network of three double agents for MI5. He passed misleading intelligence to the German Abwehr. The other example was also a British agent in World War II - the Spaniard Juan Pujol, codenamed Garbo, who was so successful in deceiving the Germans on behalfof the Allies that he was decorated by both sides. [extracted from Ultimate Spy]