Double Cross: The True Story of the D-Day Spies, By Ben Macintyre

After the Second World War, a Catalan former chicken-farmer named Juan Pujol retired to Venezuela, where he taught Spanish to Shell staff and ran a bookshop. This literary destiny seems more than apt. Between 1942 and 1945, while living in a nondescript semi in Hendon with a disgruntled wife and a dull cover job at the BBC, Pujol – "Agent Garbo" to his real controllers at MI5 – contributed several decisive chapters to perhaps the most spectacularly effective work of fiction in modern history.

Together with his double-agent colleagues, some of whose stories Ben Macintyre weaves into the latest of his enjoyable and engrossing books about wartime subterfuge and deception, Garbo fashioned a "limitless, multi-character, ever-expanding novel". It was designed to deceive the Germans about every aspect of the preparations for D-Day. From 1942, every "German" source in Britain worked for the Allies, with the bizarre multi-national band of Double Cross agents under the management of the XX Committee and its dry, cricket-fanatic chief John Masterman.

By June 1944, when the double-crossers achieved what Macintyre calls their "world-changing triumph", Operation Fortitude had managed to persuade the German high command that the Normandy landings were merely "a starter before the main course". That, the Wehrmacht believed with "unshakeable" conviction, would take the form of a massive invasion of the Pas de Calais across the narrowest point of the Channel by an (entirely fictional) First US Army Group under General Patton, with side-order attacks aimed at Norway and the Atlantic coast around Bordeaux.

Did this intricate tissue of fibs help to win the war? The military evidence looks compelling. A month after D-Day, 22 German divisions were still held back from the Normandy front. An order to move the formidable 1st Panzer Division there was rescinded specifically because Garbo, in his florid "rococo" despatches, warned that Normandy would be a diversion.

Some historians remain sceptical about the value of Second World War deception. They argue that, with Allied air superiority, the Eastern Front crumbling and the German officer elite in near-open revolt against Hitler, the D-Day forces would have prevailed without the rich diet of disinformation fed to Berlin by the agents. Writers about espionage (Macintyre included) feel a natural affinity for keyboard warriors who spin colourful yarns that command assent, and tend to overegg their tasty pudding.

In spite of these caveats, Garbo and his deeply eccentric colleagues undoubtedly saved countless lives. In his history of D-Day, the level-headed Antony Beevor writes that Fortitude "proved more effective than the Alllies had ever dared imagine". Macintyre's gripping yarns may bring no major revisions to our map of the secret war: Joshua Levine's book on Fortitude appeared only last year. But they do show with captivating panache the debt that Allied victory owed to these oddball storytellers, who wove around the actual army that liberated Europe a "bodyguard of lies".

In addition to Garbo, Macintyre traces the flights of fantasy foisted on their German handlers by Dusko Popov ("Tricycle"), the charming Dubrovnik-born spiv and seducer; by Elvira de la Fuente Chaudoir ("Bronx"), the bisexual Peruvian socialite and gambler; by Lily Sergeyev ("Treasure"), the volatile French-Russian agent who almost scuppered Fortitude because the British terminated her beloved dog Babs; and by the Polish undercover specialist Roman Czerniawski ("Brutus"), a tough pro among this cast of high-maintenance amateur primadonnas.

But did the Abwehr (German military intelligence) really swallow all the fabrications that MI5's shadow warriors fed them, prudently mixed with non-essential truth? As Macintyre's tale unfolds, we come to ask how much this notoriously "disloyal" weak link in the Nazi chain tacitly connived in strategic blunders that would hasten the downfall of the Reich. Messages intercepted at Bletchley Park by the Enigma code-breakers prove that Garbo and friends apparently retained their kudos and credibility until D-Day and beyond. However, a suspicion remains that anti-Nazi officers – right up to Alexis von Roenne, who digested and presented intelligence for Hitler - had rumbled the Double Cross fantasy and played along with it.

For all its splendidly weird ploys and feints (spy-pigeons figure frequently), Macintyre's book culminates in a stirring account of old-fashioned courage. "Worldly, cynical, dissolute", the German case-officer Johnny Jebsen - Popov's handler - was a louche playboy, high-society fraudster and PG Wodehouse fan. Always covertly anti-Nazi, he had been "turned" by MI5 in 1943. In April 1944, Jebsen was kidnapped in Lisbon and spirited to Berlin. It seems that anti-Hitler officers feared he might unwittingly derail their plans to kill the Führer.

Once in the capital, though, pro-regime interrogators took charge. He was taken to the Gestapo prison. On the eve of D-Day, Jebsen – whom everyone assumed would instantly crack and squeal – could have wrecked Fortitude. Yet, reduced by weeks of torture and maltreatment to a just-living skeleton, he said not a single word. Macintyre salutes the astonishing bravery of a hedonist-turned-hero. And this ramifying network of improbable, often comic, fraud and fiction reaches its fitting climax in a Gestapo cell, with a Wodehouse addict – broken in body, but not in spirit - who clung at death's door to the Code of the Woosters: "Never let a pal down".