Previously classified MI5 documents, which have been released by the Public Records Office, chart the success of the Double Cross System.

This involved the feeding of misleading information to the Nazis via agents whom Berlin believed were working for Germany.

Germany did not rush tanks to Normandy for two days

The system's greatest triumph was Operation Fortitude, which involved convincing the Germans that the Normandy landings (Operation Neptune) were just a diversion for a major attack further north.

The deception persuaded the Germans not to rush tank divisions to Normandy for many days, giving the Allied forces battling to establish a bridgehead an invaluable breathing space.

Success for Garbo

The MI5 documents show that Fortitude was a personal triumph for the most successful double agent run by the British, Juan Pujol Garcia - code-named Garbo.

Garbo, a Spaniard whose existence was made public in MI5 files released in January, was so successful that he was awarded both the German Iron Cross and the British MBE.

The documents show that he sent a message to German intelligence three days after the start of the Allies' offensive, suggesting that the real invasion force was still being held in reserve in England.

In Spanish, Garbo told the Germans: "From the reports mentioned it is perfectly clear that the present attack is a large-scale operation but diversionary in character."

Decoy suggestion

He suggested that the landings were meant "to draw the maximum of our reserves to the area of operation, to retain them there so as to be able to strike a blow somewhere else with assured success.

"The fact that these concentrations (of forces) which are in the east and south of the island (Britain) are now inactive means that they must be held in reserve to be employed in the other large scale operation," he suggested.

Hitler believed his reports that the larger strike was planned for Calais.

The effectiveness of Garbo's work was demonstrated by the minutes of the Twenty Committee - which oversaw the Double Cross System and took its name from the Roman numerals for twenty, XX.