This weekend the extraordinary feat of the D-Day landings will be commemorated, with due recognition for the raw physical courage and logistical skill required to land 160,000 soldiers on the beaches of Normandy. But there is one actor in the D-Day epic who is unlikely to receive the credit he deserves, even though without him the effort might well have failed. He was not a soldier. As the citizen of a neutral country, he could have sat out the war. His was not a battle of bombs and guns, but of imagination and words. He spent most of the war in a house around the corner from Hendon Central Tube station.

He was brave, gentle, brilliant and also slightly potty. His name was Juan Pujol García, a Spanish spy codenamed “Garbo”. Pujol never went near the Normandy beaches, but he played a pivotal role, saving countless lives by helping to stave off a German counter-attack. Yet there is no memorial to him, his name is largely unknown and he died in obscurity in Venezuela 20 years ago.

Bald, bearded and bespectacled, Pujol was a most unlikely wartime hero. Born in 1912 to a liberal, middle-class Catalan family, he contrived to fight for both sides in the Spanish Civil War; deserted; happily admitted that he had never fired a gun, and emerged with a ferocious hatred of Nazism.

Three times he approached the British authorities in Madrid offering to spy for Britain. Rejected, he offered himself to the Abwehr, German military Intelligence, intent on betraying them, and was taken on.

Telling his new spymasters he was heading to Britain, he hopped over the border to Lisbon and began sending a stream of fictitious reports to the Germans, with information culled from guidebooks and magazines. Pujol had never been to Britain, and it showed. His reports were full of elementary mistakes - including the intriguing notion that “there are people in Glasgow who will do anything for a litre of wine”. His German handlers, however, were utterly convinced by him.

Meanwhile, Pujol's messages were being picked up by Britain's codebreakers. Who was this German agent, operating undetected in Britain, who seemed to know nothing about the place? Finally, in 1942, after another approach to Allied Intelligence in Lisbon, Pujol was identified as the mysterious German spy. He was whisked to Britain and put to work as a double agent.

His first codename, “Bovril”, was soon changed to Garbo, in recognition of his astonishing acting talents in the service of the Double Cross System, the systematic feeding of misinformation to the Germans that remains the most remarkable intelligence coup of this or any war.

Pujol's messages to his Nazi handlers were flights of pompous poetry. He never used one word where eight would do. He assembled no fewer than 27 sub-agents, sources and informants. They were a motley crew, including Welsh Aryan supremacists, communists, Greek waiters, disaffected servicemen and crooks. The only thing they had in common was their non-existence.

In the run-up to D-Day, Garbo took centre stage for his finest performance. The deception plan covering the invasion was codenamed Fortitude; its aim to persuade the Nazis that, instead of attacking Normandy, the main thrust would be into the Pas de Calais. A vast fake US army was assembled in Kent, wireless traffic was confected and hints were dropped to less than reliable “neutral” diplomats. Many strands of deception were woven into Operation Fortitude, but none was more important than the double agent system, and of these none was more vital than Garbo.

From the MI5 safe house in Crespigny Road, Hendon, Pujol fired off more than 500 radio messages between January 1944 and D-Day, a fantastic web of deceit from his posse of bogus agents. The deception was astonishingly successful. On the day before D-Day Garbo warned the Germans that an attack was imminent, too late for the enemy to respond, but early enough to boost Pujol's status with the Nazis still further. Three days after D-Day Garbo was still warning his German handlers that the attack on Normandy was a feint and the real assault would come near Calais.

Throughout July and August 1944, the Germans kept two armoured divisions and 19 infantry divisions on the Calais coast, waiting for an attack that never materialised and giving the Allies time to reinforce the Normandy bridgehead.

Had Rommel used those divisions to counterattack in Normandy, the tide of war might have turned. But Pujol's masterful deceptions held firm and the Nazis never rumbled him. Six weeks after D-Day he was awarded the Iron Cross by order of the Fuhrer. He was simultaneously awarded the MBE, in secret. After the war, Pujol was spirited away to Caracas, where he died in 1988.

Pujol is a reminder that war is fought with brains as well as bullets and that not all war heroes wear uniform. D-Day was won by Churchill and Roosevelt, by the thousands who fought and died at Normandy. But it was also won by an eccentric Spanish civilian with a vivid imagination who wanted, he said, to make a contribution “to the good of humanity”.

Today our leaders gather in Normandy. I will be making a small alternative pilgrimage to a nondescript house in Hendon, to remember a different sort of war and a very different sort of warrior, who fought D-Day in an upstairs room, telling wonderful lies.

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