Spies have made an extraordinary impact on the history of the 20th Century, but fourteen in particular can be said to have been demonstrably important. As one might expect, few are household names, and it is only with the benefit of recently declassified files that we can now fully appreciate the nature of their contribution.

The criteria for selection have been the degree to which each can now be seen to have had a very definite influence on a specific course of events, either directly, by passing vital classified material, or indirectly, by organizing or managing a group of spies. Those selected were active in the First World War, the inter-war period, the Second World War, the Cold War and even the post-Cold War era.

These include Walther Dewé who formed a spy ring in German-occupied Belgium during the First World War. This train-watching network, known as 'White Lady', reported on German troop deployments and possible weaknesses in the German defenses. Extending its operations into northern France, the ring provided 75 per cent of the information received by GHQ, British Expeditionary Force. By the time of the Armistice in 1918, Dewé's group had a staggering 1,300 members.

Olga Gray, the 27-year-old daughter of a Daily Mail journalist, was employed as a secretary by the Communist Party of Great Britain. In 1931 she undertook a mission for MI5 to penetrate the organization and discover its secret channel of communication with Moscow. Gray learned that the Party's cipher was based on Treasure Island and this breakthrough enabled the Party's messages to be read by Whitehall cryptographers.

Renato Levi, an Italian playboy, was the longest-serving British agent of the Second World War and is credited with creating the concept of strategic deception. While operating in Cairo as a double agent working for the Abwehr and the British he was instrumental in misleading the Axis about Allied strength across the Middle East and helped Montgomery achieve his victory over Rommel's Afrika Korps at El Alamein. So successful was Levi in this and other deceptions, he was employed to persuade the Germans that the D-Day landings in Normandy were a diversionary feint, in anticipation of an invasion in the Pas-de-Calais.

These, and other surprising stories, are revealed in this fascinating insight into a secret world inhabited by mysterious and shadowy characters, all of whom, though larger than life, really did exist.