By day they were ordinary civilians — from dentists and clergymen to gamekeepers and roadmenders – in a Britain gripped by fear of imminent invasion by Hitler’s blitzkreig troops.

The only clue to their alter egos might have been the pieces of paper in their pockets – informing any police officer suspicious of their behaviour “to ask no questions of the bearer but phone this number”.

But new details have now emerged of the highly secretive role played by a “resistance” army of fit young men and women chosen as would-be saboteurs and spies in the event of a German landing.

In the dark days of 1940, the unit grew to about 6,000 members, who knew little of each other and operated in small guerrilla groups. Recruited to disrupt a German occupation force – including roles such as blowing up tanks, lorry parks and communications – the teams prepared by carrying out covert missions, known as “scallywagging”, at night.