The discovery, near the famous Castlerigg Stone Circle, is thought to date back to the first century and has solved a mystery spanning hundreds of years.

Historians always predicted there was a Roman presence in the Keswick area and now the underground remains of an ancient structure the size of eight football pitches has been found.

The site, two miles east of the town, was almost certainly a base for soldiers campaigning north of the Border or resting on their return.

The remarkable chance breakthrough came as a team of volunteers working with the Bassenthwaite Reflections programme were searching for a second stone circle or 14th century castle at the prehistoric Castlerigg site.

Armed with magnetometers instruments which can detect buried walls the team stumbled on a giant enclosure which experts say is probably a missing link in a jigsaw plotting the Roman occupation of Cumbria.

Leading the search, archaeologist Mark Graham said he thought there was little doubt that the 200m by 200m find, with interesting curved corners, was a temporary camp, capable of holding large numbers of troops.

He explained: “It could have been an important part of the first push to ‘Romanise’ the area, perhaps as early as 70AD , a militarisation that extended across the county for 300 years. It possibly serviced campaigns into Scotland and acted as a base for soldiers heading north, or withdrawing.”

English Heritage has been informed and while there are no immediate plans to organise a formal dig, Mr Graham said it would be the only way to accurately date the structure. Further exploration could also reveal important artefacts.

“There is quite literally nothing to see above ground,” he said. “In fact, the land was ploughed until 30 years ago and is now used for hay and grazing. But standing on the site, it’s clear to see why it was chosen.

“In sight of Castlerigg Stone Circle, which was already 3,000 years old at the time of the Roman occupation, the elevated position was strategically well placed for defence. It also has lovely views over Bassenthwaite and to other Roman camps at Troutbeck.”

Mr Graham paid tribute to the unstinting work of volunteers. He said there was still a great deal of work to do in analysing the results and assessing the full implications of the discovery.

Mark Cockbain, of Rakefoot Farm, said his family owned and farmed the land and he was thrilled.

He said: “An aerial photograph by Stuart Holmes revealed a crop mark which I thought might have been the lost manor of Castlerigg. “That’s what the volunteers were looking for, or a possible second stone circle.

“What they’ve found is amazing,” he said.