This MI5 advice on spy-spotting, which was given to the Home Guard, is among previously classified documents which have been released by the Public Records Office.

Another giveaway sign that a German had parachuted into Britain was that the spy would probably be under 35 - "landing from a parachute is a young man's job".

The infiltrator may have been slightly injured, so the Home Guard were told to watch out for scratches and limping.

A German probably would not have known where he was, so questions such as "Where are you?", "Where are you going to?", and "Where do you come from?" were suggested by MI5.

The spy would almost certainly be wearing civilian clothes, said MI5, "but they might have a queer cut".

Other potential giveaway signs were that he may have been carrying a wireless transmitter, foreign chocolate, food with paper showing its foreign origins, or foreign matchboxes.

Amery's anti-Semitism

Other documents reveal that one of Britain's most notorious wartime traitors was motivated by a deep hatred of Jews and Bolsheviks.

John Amery, the son of Leo Amery, the Secretary of State for India in Churchill's government, was executed for treachery in December 1945.

The previously-classified documents tell how he made pro-Nazi broadcasts to German-occupied countries and spoke of the threat posed by Bolshevik Russia.

One of Amery's anti-Semitic outbursts came in a May 1944 lecture.

"At present ... the government of England have associated themselves with Jews to acquire the wealth of the world," he said.

An MI5 report declared his conduct was clearly treacherous: "A jury trying any such case would return a verdict of guilty without leaving the box."