Juan Pujol, codenamed Garbo, was one of British intelligence service MI5's most prized wartime assets, tricking Berlin with elaborate misinformation from a made-up network of sub-agents.
The Hitler regime never discovered Pujol's deception and even awarded him the Iron Cross for his services, while he was also honoured by Britain.
In perhaps his biggest success, he helped mislead the Germans about the D-Day landings on June 6, 1944.
Afterwards, he persuaded them that the Normandy landings were a diversion before a bigger operation in the Pas de Calais area of northern France, further along the coast.
Pujol told them "that the present attack was a large scale diversionary operation for the purpose of establishing a strong bridgehead in order to draw the maximum of German reserves to the area of operations and to retain them there in order to be able to strike a second blow with ensured success," his case officer Tomas Harris wrote in a secret memo dated June 13th, 1944.
"He gave reasons as to why the second assault was likely to come in the Pas de Calais area," Harris added.
Pujol - apparently motivated by a hatred of fascism and communism rooted in his Spanish Civil War experiences - started his intelligence career feeding the Germans false information about Britain while living in Lisbon.
His information was based on sources like "a Blue Guide (tourist guide), a map of England, an out-of-date railway timetable," according to an MI5 memo dated July 12th, 1943.
"Fortunately, he has a facile and lurid style, great ingenuity and a passionate and quixotic zeal for his task," the memo added.
Even fooled his wife
Despite an initial rejection by Britain, Pujol was eventually recruited to act as a double agent and smuggled to London in April 1942.
However, his wife Araceli struggled to cope with their new life.
One incident showed how her husband was even prepared to put his ingenuity to work on her to protect his position.
On June 21st, 1943, she telephoned Harris threatening to leave Pujol and go to the Spanish embassy in London to disclose her husband's work to the Franco regime unless she could go home.
The couple had rowed after he refused to take her to a dinner where there would be embassy staff.
The next day, after another call, an MI5 officer went to the family's north London home and found her "sitting in the kitchen with all the gas taps turned on".
Describing Araceli as a "highly emotional and neurotic woman", Harris described in a report six days later how Pujol hatched a plan to convince his wife that he had been arrested due to her behaviour.
This was designed to calm her down and prevent similar actions in future. She was even taken to visit him in an interrogation camp before apologising two days later and vowing "never to perform any action which might compromise my husband's work" again.
"The extraordinary ingenuity with which he has conceived and carried through this plan has perhaps saved a situation which might otherwise have been intolerable," Harris wrote.
Pujol moved to Angola after the war, where his death was faked before he went on to build a new life in Venezuela. He died there in 1988.
Leading British historian Christopher Andrew, MI5's official biographer, described him as "the greatest double agent of the Second World War and possibly of the entire 20th century" in a new podcast released with the documents.
The information came from classified MI5 files made public by Britain's National Archives.
By Katherine Haddon / AFP