Spaniard behind the D-day landings

Juan Pujol Garcia, operating under the codename Garbo, was trusted as a valuable spy by the Germans.

During WWII they received hundreds of radio transmissions from him detailing the British war effort.

"Information" to Germany was fabricated

But the London-based agent's information was entirely fabricated.

Perhaps his most important coup was to persuade the Third Reich's top commanders that the 1944 D-day invasion was merely a decoy for a major assault further along the coast near Calais.

The Germans swallowed Garbo's "information" hook, line and sinker, and reinforced the area they were told would come under attack from two fictitious Anglo-American armies.

June 1944's Operation Overlord - as the D-day landings were officially called - was able to go ahead with the bulk of the Germans' strength placed in the Pas de Calais.

Despite their defeat at Normandy, Hitler's generals continued to place faith in Garbo, believing him to be an impassioned fascist.

The son of an industrialist, who fought for Franco during the Spanish Civil War, Garbo had been recruited by MI5 in 1942.

He had originally been recruited by German spymaster Karl Kuehlanthal, whom he had fooled with fake reports allegedly filed from a hideout in England.

Constant stream of misinformation

But Garbo's bulletins on British troop and shipping movements were made from his home in Portugal, and were gleaned entirely from an Ordnance survey map of Britain, a Blue Guide, a Portuguese publication called the British fleet - and a dictionary of military terms.

Later, working for MI5, he invented a network of 27 "agents" and fed his Nazi spymasters a constant stream of misinformation on British war preparations.

A drunken RAF officer in Glasgow and a Communist-hating War Office linguist were amongst the characters the Spaniard invented as sources for his information.

His audacity and creativity lead to his codename being changed from Bovril to Garbo after his British bosses said he had proved himself to be "the greatest actor in the world".

The MI5 file - an official history of Garbo was written by his case officer Tomas Harris - and has now come to light as more top secret files are released by the secret service.

They show how Garbo's information to the Germans included fictitious tales throughout the war, including the existence of a massive arms dump under Chislehurst in south-east London.

Awarded the Iron Cross

German commanders were so convinced of the existence of the depot, linked across London by a subterranean train network, that they considered a plan to send saboteurs to it blow up.

"Garbo's reports formed the backbone of all German Intelligence appreciation on which vital operational decisions were taken," wrote Harris at the end of the war in the autumn of 1945.

And praise had also been forthcoming from his Nazi commanders - he was awarded the Iron Cross for his services to Germany, with the approval of Hitler.