Did Hitler hatch bizarre plot to capture King George VI?

York-based English Heritage military specialist Roger Thomas found a file in the public records office at Kew showing that Germans appeared to have built a network of aerodromes on farms close to the royal family’s Sandringham Estate in Norfolk ready to land Hitler’s army.

Sandringham is only half an hour’s drive from where the first of the German Operation Sea Lion invasion forces would have landed in 1940, secret files have revealed.

Police and MI5 failed to notice the airstrips, occupying 2,000 acres of farmland from 1936, despite chicken huts apparently being laid out in the shape of swastikas.

The astonishing plot resembles two of the classic novels by Jack Higgins - The Eagle Has Landed and To Catch A King.

A recently unearthed Air Ministry dossier reveals one landing spot was made up of two farms at Sporle, near King’s Lynn, 16 miles from Sandringham. Intriguingly, it was close to where the Air Ministry later built airfields between 1942 and 1943.

The royal lawns were also flanked by another hidden aerodrome made up of another two farms 25 miles away at Guestwick.

Mr Thomas said: “I was absolutely astonished. There was nothing in the press about it or anything in the written history.

“There were two of the barns at Sporle, so who knows if Sandringham was a target. But the whole story is quite intriguing.

“The Germans used a similar trick when they invaded the Low Countries using landing grounds on pre-arranged sites before their main force arrived.”

According to the files, the fields were all at least 200 acres, bare of crops, and “rolled hard”.

Bomber Command noted in the dossier that the landing strips appeared “suitable to land and take off heavy aircraft in all directions”.

The Air Ministry warned: “Consider immediate investigation of site and neighbouring population essential.”

The Army was called in to ensure the fields were “camouflaged and obstructed” for the remainder of the war but the dossier does not say how.

However, recent aerial photographic interpretation carried out by the National Mapping Project in Norfolk shows that they were blocked by a grid of trenches and mounds of earth, deliberately spaced to tip over a German Junkers JU52 transport aircraft.

The spy masters had hoodwinked Dutch farm workers into building the barns. The farmhands were all released from police custody after a few weeks.

The steel was supplied by the same company that supplied materials for the Bellman air hangar, 400 of which were deployed in Britain during the war.

Many of the Dutch barns still survive today and much of the hastily applied camouflage paint has flaked off to reveal the original red roof tops. By the time the RAF spotted the bases from the air, England stood on the brink of invasion and Hitler’s bomber and paratrooper squadrons were ready to fly.

MI5 then found a nest of enemy spies had been building the airstrips from 1936 onwards by flattening fields and putting up hangars disguised as Dutch barns.

No one noticed - including Norfolk Police - until their Chief Constable gave orders to round up the plotters.

Each airstrip was at least 200 acres – the equivalent of about 100 football pitches. The grounds of Buckingham Palace cover 40 acres.

Every barn had a red roof so pilots could spot where hedges and ditches had been removed to create landing areas for bombers and Junkers 52 transport planes.

The spies had finished nine airfields in Norfolk and one near Woking in Surrey before the authorities realised they had been fooled by a bunch of friendly Dutch farmers.

They were recruited in Holland and imbedded with their families in the fens, where they were supposed to be restoring derelict farmland with Dutch drainage expertise.

By 1940, the RAF was scouting East Anglia to build new airfields of its own.

But every suitable site with firm ground had one, two or even three of the Dutch-style barns on it.

The pilots also noticed that the large meadows were not waistdeep in corn like the surrounding countryside.

When MI5 was called in to investigate they found all ten sites were operated by the same business: the Dutch-owned East Anglian Real Property Company.

The Dutchmen claimed they needed the supersize barns to house corn and specialised grain machinery.

But when the doors were slid open the buildings were empty and there was nothing growing on the farms.

One night in 1940, police arrested the manager, his wife, and the farm workforce.

Their children were found billets in local houses and later deported while the spies were jailed.