A 50-yard stretch of Offa’s Dyke, which has been left undisturbed for 1,200 years, has been flattened with a bulldozer in a move described by experts as similar to 'driving a road through Stonehenge'.

The 8th Century monument runs for 82 miles along the border between England and Wales and is a Unesco World Heritage site with protected status.

Police, along with Cadw, the Welsh Government's historic environment service, are now investigating claims that a section of the dyke was flattened by a large digger clearing scrub and weeds near to the A5 between Llangollen and Chirk.

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One witness told the Daily Mail: "I don’t think the men who cleared the hedge and weeds realised the significance of what they were doing.

"A lady from Cadw who came to inspect it... said what they had done was like driving a road through Stonehenge."

It's a criminal offence to damage Offa's Dyke under the Ancient Monuments and Archaeological Areas Act 1979. Anyone who is found to have damaged the 8th century monument can face a £5,000 fine or a six month jail sentence.

Jim Saunders of the Offa’s Dyke Association said: "We had a report that quite a well-preserved part of the dyke had been damaged by the new landowner."

"I have been living and working with Offa's Dyke for 25 years and if the reports I have had are correct then this is the worst example of destruction of the Ancient Monument that I can recall."

According to reports the owner of the field claimed he had no idea that the site was of historic importance saying he had "lived here all my life and I've never heard of Offa's Dyke."

Offa was the King of Mercia from 757 until his death in July 796. His kingdom encompassed the area between the Trent/Mersey rivers in the North to the Thames Valley in the South, and from the Welsh border in the West to the Fens in the East. He also at one point controlled Kent, East Anglia and Lincoln.

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Offa's Dyke is a linear earthwork which follows the English/Welsh border. It has a ditch and rampart and runs for 82 miles along the border between England and Wales.

The section believed to have been damaged is near to the famous Pontcysyllte aqueduct, which is a 126ft canal boat crossing high above the River Dee in the Vale of Llangollen, north Wales.

A spokesperson for Cadw said: "We are aware of the reported damage to a section of Offa's Dyke which is a nationally important ancient monument protected by law.

An investigation is currently underway to determine whether or not an offence has been committed. The police have been informed."

In October of last year Lonely Planet named Offa's Dyke one of the 10 best wall walks in the world alongside the Great Wall of China, the 17th century walls of Quebec City and the Berlin Wall.

A North Wales Police spokesman said: “We have been made aware by Cadw and are waiting for their assessment.”