New secret files have revealed a mystery MI5 agent who foiled British Nazi spy network with fake Iron Crosses.
From a clandestine HQ in an antique shop basement, more than 100 spies used invisible ink to pass on secrets which could have changed the outcome of the war.
Maps of military defences, plans for experimental jet fighters, reports of amphibious tanks and anti-radar technology - all were passed on to a man the spies knew as an undercover Gestapo officer.
In fact newly released papers from the National Archives have now revealed their spymaster as a loyal MI5 agent known as "Jack King", the Mirror reported.
Thanks to him, the secret service was controlling every move made by Britain's enemies within.
Some of the spies were foreigners with German relatives who were living in Britain when the war began.
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MI5 even drew up plans to issue them with secret badges to be worn in the event of an invasion, supposedly to identify them as friends to the Germans, but in reality to enable a swift round-up by the police.
The agency also acquired replica Iron Cross medals to reward members of the network for their loyalty while adding further proof that they were working for the enemy.