Newly declassified documents have revealed that Britain's security service issued a covert alert in December 1944 warning of a possible Nazi plot to kill members of the Royal Family or Prime Minister Winston Churchill.
The intelligence assessment, prepared jointly by MI5 and the Air Ministry, raised fears that Germany could deploy elite airborne troops on English soil with assassination as their objective.
This warning came just days after Adolf Hitler launched his unexpected military offensive through the Ardennes forest in Belgium, an assault that would become known as the Battle of the Bulge and caused considerable alarm among Allied forces.
According to the intelligence report, the Germans possessed the capability to mount an assault force comprising as many as 500 paratroopers, transported aboard twelve large Junkers Ju 290 aircraft.
The assessment also identified an alternative method of attack involving seized American B-17 Flying Fortress and B-24 Liberator bombers flown by the Luftwaffe's specialist KG200 unit.
"It is possible," the report stated, "that they might use these, with the Allied markings, for a special operation by day or night."
The KG200 squadron was described as a special flying unit typically tasked with dropping agents behind Allied lines, known to operate captured foreign aircraft of various types.
The report noted that deploying fewer aircraft would improve their chances of evading British defences.
Sir Samuel Findlater Stewart, a senior civil servant with established MI5 connections, conveyed the intelligence assessment to Gen Colin Callander, deputy chief of Home Defence Forces, on 20 December 1944.
"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," Sir Samuel wrote.
The warning prompted an immediate strengthening of protective measures around both the Royal Family and Churchill.
Gen Sir Harold Franklyn, Commander-in-Chief of Home Defence Forces, was consulted the following day regarding the appropriate response.
Gen Franklyn's position was unequivocal regarding the protection of Britain's most important figures.
Gen Callander relayed his superior's view 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."
Beyond maintaining heightened security, military commanders acknowledged their options were limited.
"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," Gen Callander added.
Despite the concerns, the German paratroop assault never took place.
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