Adolf Hitler's plan to invade Great Britain received help from an unexpected source - young Boy Scouts.

The Nazi leader and his henchmen in the Third Reich wanted to subjugate the nation and convert its lands into a vast potato field, interspersed with factories for manufacturing walking sticks.

In order to draw up the detail for Operation Sea Lion - as the invasion was known - the Nazis plundered information contained in hundreds of postcards sent home by German Scouts during trips to the UK.

The Nazi war machine gleaned information about possible landing sites, military targets and infrastructure including transport networks from the homely missives.

Eight files were compiled with each one focused on a different region. There was a list of common phrases for soldiers to use when communicating with natives.

An image of rolling countryside in Devon notes "typical hedgerows." Another card depicting the beach at Folkestone in Kent notes the presence of a railway nearby.

Red and purple ink were used to mark places such as police stations, railway stations and factories.

The postcards have been brought together in a volume entitled Military Geographical Information about England. It is set to go under the hammer at an auction.

Scenes on the postcards include views of harbours in Newlyn and Porthleven, as well as Millwall docks in east London.

Max Hasler of auction house Dreweatts explained the sinister use to which could be put by the Nazis: "These pamphlets are very, very detailed. They contain things like composition of the beaches, where different electricity points are, where pump stations are, where the hospitals, the schools, the prisons are.

"And it's all building up this very, very detailed picture of Britain. Both, in terms of weak points and once the proposed invasion has taken place, the Germans can move in and have a place up and running again as part of this huge German war machine."