The Security Service were worried the Nazis would target the Royal Family at the end of World War Two, declassified files have revealed. MI5 issued a warning that Germany might try to assassinate certain members.

The warning was issued in December 1944, after Hitler launched an offensive in the Ardennes Forest in Belgium. This panicked the Allied Forces, and MI5 was concerned about the possibility that the Nazis would drop elite paratroopers into the UK, with a plot to assassinate senior royals and Prime Minister Winston Churchill. An intelligence report written up on December 20 was given to senior civil servant Sir Samuel Findlater Stewart. He wrote to Gen Colin Callander, deputy chief of Britain’s Home Defence Forces, saying: “Through the courtesy of MI5, with whom, as you may know, I have certain contacts, I had the opportunity today to read an appreciation, prepared I understand by the Air Ministry, on the subject of the enemy’s ability to mount parachute raids on this country.”

He told the general that, according to MI5, a raid could include up to 500 paratroopers brought in by a dozen Junkers Ju 290 transport aircraft.

The Nazis could reportedly have instead used captured American B-17 Flying Fortress and B-24 Liberator bombers, operated by the Luftwaffe’s elite KG200 squadron.

“It is possible,” the report stated, “that they might use these, with the Allied markings, for a special operation by day or night.

“KG200 [‘Special’ flying unit which is generally employed for dropping agents over Allied territory] is known to handle foreign aircraft of all types, including Fortresses and Liberators.

“The fewer the number of aircraft employed, the better chance they would have of eluding our defences,” the report continued.

Security for both the Royal Family and the Prime Minister was increased as a response, after Gen Callander consulted with Gen Sir Harold Franklyn, the Commander-in-Chief of Home Defence Forces.

Gen Callander said: “[Franklyn] feels that it would be very wrong to take any risks over the guarding of the Royal family and the Prime Minister, and London District and others concerned have been told that on no account must they relax their precautions in this respect.

“Apart from this, I do not think that there is anything we can do at the present time except to keep the possibility of some raid constantly in mind so that we shall not be caught on the hop if it does in fact occur.”

A raid by Nazi paratroopers never took place.