A previously unkown Roman fort has been found at Calstock in Cornwall, one of only a handful of sites giving evidence of Roman presence in the county, and the first found close to a silver mine.

Archaeologists from the University of Exeter say the site may be evidence the Romans mined tin in the county.

The hill-top site where the first-century fort is in an area known to have been involved with medieval silver mining in the 13th and 14th centuries.

University archaeologists became interested in the site when they found references in medieval documents to the smelting of silver at the old castle and next to the church in Calstock.

A geophysical survey - similar to an underground X-ray - clearly showed the outline of a feature that is a very similar shape to another Roman fort recently found near Lostwithiel, also in Cornwall.

The team started digging and uncovered the unique and instantly recognisable shape of a Roman military ditch, confirming their find as a Roman fort.

The fort, which measures about 80 square metres, was probably used as soldiers' barracks, workshops and stables, and is very well preserved.

It stands just a couple of miles away from a silver mine and has the remains of furnaces - indicating smelting activity.

University of Exeter archaeologist Dr Stephen Rippon said it was an exciting find, which could yield important information in piecing together more about the Romans, who invaded Britain in 43 AD.

"When I first saw the results from the geophysical survey, suggesting the outline of a Roman fort, I could hardly believe my eyes," he said.

"As an archaeologist it is so rare to find something so significant, which was previously entirely unknown.

"It's a very exciting discovery."

"It's possibly a coincidence that the Roman fort and army were located so close to this mine but we do know elsewhere in Roman Britain that the Roman army were involved in mining minerals.

"Romans knew about Britain's rich mineral wealth and there's even evidence of tin being exported to Europe even before the Roman invasion."

However, it is not known whether the Romans mined silver in the country, but radiocarbon dating tests on the fortress are being carried out to allow the team to date the industrial workings at the fort to discover whether the Romans were smelting silver.

Results are expected within the next few months.

"If we find this to be the case it could possibly be the first example of Romans silver mining in Britain," Dr Rippon said.

"They would have dug up the ore, transported it back to camp and that's where they would have done the smelting."

It is thought that the precious metal would then have been transported back to Italy where it would have been minted into coins for use within the empire.

"The Roman army only stayed in the South West for a few decades after the Conquest before moving on to Wales," Dr Rippon said.

"This find could help us to understand whether they were merely keeping watch over the locals or were actually interested in exploiting commercial opportunities in the region.

"The discovery could therefore further our understanding of the rich history of mining in the county.

"It's only the third Roman fort in Cornwall.

"We know very little about what the Roman Army was doing down here.

"It could go some way to further explaining what attracted the Romans to Britain."

The team of excavators, led by University of Exeter research fellow Chris Smart, has also dug up pottery, believed to be from the first century AD.

The research project was funded by the Leverhulme Trust with additional support from the University of Exeter.

The two other known sites of Roman forts in Cornwall are also in the South East of the county.

One was discovered last year near Restormel Castle, Lostwithiel, and the other is at Nanstallon, near Bodmin.

Both sites are close to mineral deposits in areas which are associated with tin mining.