MI5 HQ manipulated Nazi spies into their fold

One of the documents describes him as "a typical Welsh "underfed" type, very short, bony face, ill-shaped ears, disproportionately small for size of man, shifty look."

Snow had been given a transmitter by the Germans and at the outbreak of war was arrested and taken to London's Wandsworth Prison.

Under MI5 control there, he continued his double agent work, feeding through harmless and also downright misleading information to the Nazis.

One of the tasks set by his German masters was to find out how easy it would be to put bacteria in the British water system.

After one visit to Germany he returned with explosives hidden in talcum powder, pens and other sabotage material.

Eddie Chapman, alias Zigzag, was considered the most flamboyant double agent during the war.

The Nazis even gave one double agent a medal

An ex-con recruited by the Germans when they captured Jersey, he was highly trained as a saboteur.

However, as soon as he was parachuted into Britain he walked into a local police station, declaring he was a loyal British citizen but also a German spy who wanted to work for MI5.

MI5 took him on and created fake pictures and news coverage of a raid on the Mosquito bomber plant, a plane the Nazis were in awe of and wanted Zigzag to sabotage.

The Germans were so impressed they awarded the double agent the Iron Cross.

The secret files also contain photographs by another double agent codenamed Charlie, whose work was miniaturised and sent to the Germans hidden behind postage stamps.

The British spymasters had trouble, though, from Agent Treasure, alias Lily, a Frenchwoman of Russian extraction who the records say proved to be "exceptionally temperamental and troublesome".

On one occasion she threatened to stop working for them if they did not bring over her pet dog to Britain to join her.

These fascinating files just revealed show how Britain effectively controlled the German espionage system but also how it never fully trusted its own double agents, spying on the spies who were helping Britain win the war.