He fought on both sides in the Spanish Civil War. He was awarded the Iron Cross by Hitler and an MBE by Britain. To MI5 he was known as Garbo. To the Abwehr, he was Alaric. He also went by Rags the Indian Poet, Mrs Gerbers, Stanley the Welsh Nationalist – and 24 other names. He tricked Hitler over D-Day. He was the greatest double agent in history.

But who, exactly, was Juan Pujol?

Using his intimate knowledge of Spain and his skills as a crime novelist, Jason Webster tells for the first time the full true story of the character who captured the imagination in Ben Macintyre’s Double Cross. He tells of Pujol’s early life in Spain, his determination to fight totalitarianism ­and his strange journey from German spy to MI5. Working for the British, whom he saw as the exemplars of freedom and democracy, he created a bizarre fictional network of spies – 29 of them – that misled the entire German high command, including Hitler himself. Above all, in Operation Fortitude he diverted German Panzer divisions away from Normandy, playing a crucial role in safeguarding D-Day and ending the war, and securing his reputation as the most successful double agent of the war.

Meticulously researched, yet told with the verve of a thriller, The Spy with 29 Names uncovers the truth – far stranger than any fiction – about the spy behind one of recent history’s most important and dramatic events.