MI5 spent years trying to decipher crop circles amid fears they were being used to guide Nazi bombers, secret documents from the Second World War have revealed.

The files, held in the National Archives, show how British spies suspected Nazi sympathisers were sending secret messages to the enemy using ground markings.

Among the suspected codes was a mysterious marking in a cornfield that was shaped like the letter 'G', with its tail appearing to point to a munitions factory in Glascoed, Monmouthshire.

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MI5 spent years trying to decipher crop circles, including this 'G' shape which appeared in a Monmouthshire field amid fears they were being used to guide Nazi bombers, secret files from the Second World War reveal

Other suspicious marks which showed up when British spies investigated aerial photographs were strange marks in a field near Newquay, Cornwall, and a white circle with the word 'Marden' inscribed inside it in a field near Staplehurst, Kent, while agents were also tasked with probing reports of lights in the sky.

The British security service had started looking into the marks after learning that in Poland, Holland, France and Belgium, cornfields had been cut into 'guiding marks for aircraft', with spies beating out marks to assist Luftwaffe pilots find their targets.

Fearing that a 'Fifth Column' of Nazi sympathisers were working inside Britain, MI5 dedicated agents to investigating reports of suspected enemy activity, including 'cornfield codes' spotted in fields.

The files, held in the National Archives, show how British spies suspected Nazi sympathisers were sending secret messages to the enemy using ground markings, such as these discovered in a farmer's field in 1940

MI5 started looking into the marks after learning that in Europe, cornfields were cut into 'guiding marks for aircraft', with spies beating out marks to help Luftwaffe pilots (pictured) find their targets

'They began investigating into the Fifth Column after the invasion of the Low Countries and France where German spies had used landing lights to guide them to the places where the Nazis wanted to attack,' said Dr David Clarke, senior lecturer in journalism at Sheffield Hallam University who has studied the files for his book Britain's X-traordinary Files.

'In 1940, everyone in Britain thought "we're next", and that Germany would have recruited spies in England as well who would be sending messages to the Germans and marking places out where landings could be made.

'The German Operation Sealion led people, including the police, Winston Churchill and MI5 to suspect there was an active Fifth Column and there is an MI5 file in the National Archives that's absolutely full of weird things that were brought to their attention.'

The MI5 report into the markings discovered at Little Mill, Monmouthshire, which turned out to have been made accidentally by a farmer trying to get rid of a supply of barley seed

It was feared that Luftwaffe pilots flying over Britain could be guided to targets with messages on the ground

Included in the file is the report from the marking found in the field at Little Mill, Monmouthshire in 1941.

It stated that the 33 yards long shape had been made by 'sowing barley seeds transversely through the grain', and the tail of the G was pointing towards the munitions factory in nearby Glascoed.

A SECRET CODE FOR THE LUFTWAFFE OR A FARMER'S SIMPLE MISTAKE? THE SECRET MI5 FILE INTO STRANGE MARKINGS AT LITTLE MILL, MONMOUTHSHIRE 'In May 1941 a report was made that an unusual mark was visible amongst the growing corn. Near one of the gates was a mark in the form of the letter G, some 33 yards long. This mark had been made by sowing barley transversely through the grain.' 'Air photographs were taken and it was seen that the tail of the marking pointed towards the Ordnance factory at Glascoed. The farmer, a man of good character, was interviewed, and admitted that he had sown the field himself. He explained that he had sold the field in April. 'Shortly after, having a drilling machine nearby which had a small quantity of barley seed in it, and wishing to empty it as he had to return it to the farmer from whom he had borrowed it that night, he turned his team of horses into the grain field and drilled it into the ground thickly to get rid of it. He did this because it is extremely difficult to remove the grain in the machine by hand, and to sow it was the quickest way of getting rid of it. 'He agreed to plough up this part of the field. As a satisfactory explanation had been reached, the case was carried no further.' Advertisement

'It was spotted from the air by the RAF pilot who had been given instructions to look for anything suspicious,' Dr Clarke told MailOnline.

'A farmer had left a "G" shape and it just so happened that the tail of the G was pointing towards an Ordnance factory.

'When the RAF saw that from the air they thought it was a German spy and the poor farmer was surrounded by MI5 agents.

'But there was a perfectly innocent explanation.'

It transpired the farmer had sown the barley seed in order to get rid of it, having sold his farm and needing to return a drilling seed he had borrowed.

Likewise, a marking in a field in Newquay was discovered to have been made by heaps of lime used for agricultural purposes, and the Marden sign in Staplehurst had been left behind from its days as an emergency landing ground by Imperial Airways, which eventually though a number of mergers became British Airways.

'In the files are a number of things that if people had seen them recently would be thought of as alien activity such as lights in the sky or crop circles,' said Dr Clarke.

'In 1940, people weren't pre-occupied with UFOs, they were pre-occupied with Nazis.

'The amount of time and energy that MI5 put into investigating these reports is astonishing. They had three officers full time based in the Eastern counties looking for lights in the sky they thought were agents signalling to German aircraft "this way to Norwich".'

He added: 'When people are worried, like they are now with the possibility of Islamic terrorism, they can interpret normal things as signs that horrible things are going to happen.

'That's why MI5 is interested in things like sightings of flying saucers - but they are interested in the fact there may be more grounded reasons for them.'