D-Day, 6 June 1944, the turning point of the Second World War, was a victory of arms. But it was also a triumph for a different kind of operation: one of deceit, aimed at convincing the Nazis that Calais and Norway, not Normandy, were the targets of the 150,000-strong invasion force.

The deception involved every branch of Allied wartime intelligence - the Bletchley Park code-breakers, MI5, MI6, SOE, Scientific Intelligence, the FBI and the French Resistance. But at its heart was the 'Double Cross System', a team of double agents controlled by the secret Twenty Committee, so named because twenty in Roman numerals forms a double cross.

The key D-Day spies were just five in number, and one of the oddest military units ever assembled: a bisexual Peruvian playgirl, a tiny Polish fighter pilot, a Serbian seducer, a wildly imaginative Spaniard with a diploma in chicken farming, and a hysterical Frenchwoman whose obsessive love for her pet dog very nearly wrecked the entire deception. Their enterprise was saved from catastrophe by a shadowy sixth spy whose heroic sacrifice is here revealed for the first time. Under the direction of an eccentric but brilliant intelligence officer in tartan trousers, working from a smoky lair in St James's, these spies would weave a web of deception so intricate that it ensnared Hitler's army and helped to carry thousands of troops across the Channel in safety.

These double agents were, variously, brave, treacherous, fickle, greedy and inspired. They were not conventional warriors, but their masterpiece of deceit saved countless lives. Their codenames were Bronx, Brutus, Treasure, Tricycle and Garbo. This is their story.