EVIDENCE that suggests Christianity was practised in Britain as early as the third century has been unearthed in a Roman fort in County Durham.

A silver ring, set with a carnelian stone carved with the early Christian symbol of an anchor and two fishes, was discovered during an excavation at Binchester, near Bishop Auckland.

The project co-ordinator, Dr David Petts, a lecturer in archaeology at Durham University, said on Monday that the design of the ring dated it to the third century, which was significant, because that was before the Emperor Constantine legitimised Christianity in 313.

"Along with a very similar ring, and a roof tile bearing the chi-rho symbol, both of which were found in York, it is the earliest evidence we have for Christianity in Roman Britain," he said. During that early period, the Church was keeping a low profile, so there were not many distinctive objects in existence, he said.

"Christianity went through phases: on some occasions, it was just ignored; but, on others, there were persecutions." In Rome there might be plenty of artefacts; but "out here, at the edge of Empire, it is hard to find things that are distinctively Christian." This was why the ring was so important.

Since it was a valuable item, the ring was likely to have been owned by someone of status. Because it was found in the fort, the owner may have been an officer.

The Roman unit at Binchester was originally a Spanish cavalry regiment, "but they would have become Anglicised over time. Also, as Binchester was on the main road from York to Hadrian's Wall, it would have had a constant flow of people passing through - as mucha motorway service-station as a fort."

Dr Petts hopes that the ring will eventually go on show to the public in a museum in County Durham.

The Binchester site where the ring was found has been called the "Pompeii of the north", owing to the discovery of an exceptionally well-preserved bathhouse in the civilian settlement around the fort. Its walls, which were once covered with bright painted designs, stand more than seven feet tall, and it still has its original floor, doorways, and window openings.

Dr Petts said that it had survived because locals had used it as a dump in the 300 years after the Romans' departure. Tons of domestic rubbish propped up the masonry.

"There are perhaps only three or four other buildings in Roman Britain which have that quality of preservation," Dr Petts said. "It's very rare to have walls standing to head height. It's more like study-ing a historic building thanlike digging at an archaeological site."