Mata Hari, the glamorous first World War spy shot by the French in 1917, had been under surveillance by Britain's internal intelligence agency MI5 for two years, according to secret documents released today.
The former wife of a Dutch army colonel who, while performing as a stripper in Berlin, was recruited by German intelligence, first came to the attention of British officialdom in December 1915.
Arriving at the southern English port of Folkestone on the boat train from Dieppe in France, Mata Hari - whose real name was Zelle MacLeod - attracted suspicion.
At first she told officials that she was on her way to South America to continue her career as a dancer but then admitted her destination was The Hague so she could be near her lover, Baron Van der Capellen, a Dutch Hussars colonel.
After subsequent cross-examination by an MI5 officer, the authorities could pin nothing on the spy and had to let her go.
During the second World War, a British double agent duped Hitler in a secret service plot to ensure the success of the D-Day landings, according to the released papers.
Hundreds of radio messages transmitted from London by the spy, code-named Garbo, conned Germany's commanders into believing that the 1944 Normandy invasion was an elaborate diversion ahead of a main attack near Calais.
Vital Nazi reinforcements were held back or diverted to eastern France on the basis of "information" from Garbo - a Spaniard whose real name was Juan Pujol Garcia - that two fictitious Anglo-American armies were massing in Dover.
In another revelation, MI5 considered using a "truth drug" on Hitler's deputy, Rudolf Hess, to force him to disclose Nazi secrets.
Hess was captured in 1941, during the second World War, after flying to Scotland in a bizarre and unauthorised attempt to negotiate a peace deal between Britain and Germany. The British were not impressed by his claims and he was swiftly interned in "Camp Z" - a secret location near Aldershot.
Nevertheless a War Office official did write to Capt Guy Liddell of MI5 with a suggestion for "the picking of whatever brains that gentleman may still possess".