Secret files about the Spanish MI5 agent described as "the greatest double agent of the Second World War" reveal his homesick wife nearly blew his cover, a move that might have changed the course of the conflict.

The documents released to the National Archives in Kew give a fascinating insight into the life of Juan Pujol, codenamed Garbo.

He fabricated an imaginary network of spies in the UK who the Nazis believed were working for them but who were, instead, part of a disinformation campaign orchestrated by the British.

His most significant impact was in the run up to and immediately after D-Day on 6 June, 1944.

The double agent contacted his German handlers to tell them the Normandy invasion was happening but, crucially, Garbo managed to convince his handlers in Berlin that Operation Overlord was a bluff and that the real allied invasion would happen later, further up the French coast.

He was so trusted by the Germans that they held back reinforcements from Normandy in anticipation of the "proper" assault on the Pas de Calais region which, of course, never came.

But his private life was also complicated and could have jeopardised the pivotal mission.

In June 1943, a year before D-Day, his Spanish wife Araceli, disillusioned with living in London and having her movements controlled, threatened to go to the Spanish embassy and tell all unless she was allowed to travel home to see her mother.

To stop her blowing his cover, Mr Pujol had to concoct a story with MI5 that he had been arrested by them after a "hysterical outburst" over her treatment.

After being taken to an interrogation centre and seeing him unshaven and in camp clothing, she tearfully promised to continue supporting his spying efforts.

Authorities had been so worried about her talking to officials at the Spanish embassy that police were stationed around the building to intercept her.

Despite becoming crucial to the allies, Garbo was at first turned down twice by the British secret service.

But - convinced Hitler and the Nazis needed to be stopped - he went to them in his native Spain and eventually managed to persuade MI5 to take him on.

Garbo was so successful at deceiving the Germans that they even awarded him the Iron Cross after the war for his efforts.

The British, meanwhile, gave him an MBE.

Author Nigel West managed to find Garbo in South America before he died and West co-wrote the spymaster's incredible story of espionage.

After the war Mr Pujol, his wife and three children moved to Venezuela, but his deception was not over.

Having sent his wife and family back to Spain and promised to follow them, he conspired with MI5 to fake his own death and begin a new life in South America with a new family.