General Alan Brooke, commander-in-chief of Home Forces, said that he did not have enough troops to protect RAF airfields in Scotland (Getty Images)

Military commanders feared in the summer of 1940 that Hitler planned to parachute tens of thousands of troops into Scotland as a precursor to a full-blown invasion of Britain.

Previously classified files, which have remained unseen for 75 years, reveal that the war cabinet became convinced that a Nazi invasion of northern Scotland was imminent.

Official documents from August 1940 expressed fears that the Germans planned to use newly occupied Norway to launch an incursion into the UK.

The top secret briefings were drawn up for the war cabinet, Ministry of Defence and Cabinet Office joint intelligence sub-committee.

The threat of invasion only receded after the Royal Air Force regained air superiority following the Battle of Britain between June and October of that year. In