EXCITEMENT over a Roman gravestone discovered in the centre of Lancaster has been dampened by the news that, although the artefact is barely out of the ground, Britain is likely to lose it to an overseas buyer.

Archaeologists said yesterday that the gravestone, which depicts with great clarity a mounted trooper holding a sword and the head of a man he has just killed, was a unique find.

The stone has yet to be dried, conserved and studied, but its owner — the developer on whose land it has been found — has already sought valuation advice from Sotheby’s.

Christopher Tudor-Whelan, director of Tudor-Whelan Property Holdings, which specialises in commercial investment properties, hopes to sell it in New York. He confirmed yesterday that he has been told that he can expect to sell it for “up to $100,000” (£57,500).

The gravestone, which commemorates a cavalry officer of the late 1st century or early 2nd century AD, was unearthed when archaeologists excavated land in the city centre before construction work began on a block of flats. Experts are overwhelmed by the artefact’s quality, although it is in three pieces and yet to be reassembled.

The stone, which originally would have measured 2.5m (8ft) in height, features a solar face, reminiscent of the famous Medusa head from Roman Bath, above the trooper’s head. The beheaded victim kneels on the ground, holding his sword.

Although beheading war victims was accepted Roman practice, it is thought that no such depiction of a man on horseback has been found before.

Importantly, the stone also bears an inscription that provides clues to the man to whom it was dedicated — a citizen of a Celtic tribe in northern Europe, the Treveri, which is known to have occupied an area where Belgium, Germany and France meet. The tribe was said to have provided Julius Caesar with his best cavalry.

The inscription refers to a man called Lucius Nisus Vodvilleius, or Insus, son of Vodullus. The precise name is unclear as it is abbreviated.

He served in the Ala Augusta, the Augustun cavalry stationed in Lancaster in the late 1st century. Although he had not been granted Roman citizenship, he had clearly achieved considerable status in the Roman Army.

The discovery will be published by British Archaeology tomorrow. Mike Pitts, the editor of the magazine, described the stone as “immensely exciting and seriously important”. He said: “Much of Roman sculpture is very fragmentary and, even if it is in good condition, it is not of a particularly high quality aesthetically. This is more or less complete, with a carving whose quality is superb.”

The stone is drying out now at the Lancashire County Museums Service in Preston. Mr Pitts believes that its rightful home is one of the Lancaster museums. “I can see people queueing down the block to see it,” he said.

Stephen Bull, curator of archaeology at the Museum of Lancashire, described the carving as “one of the sharpest and clearest I’ve ever seen”.

The developer confirmed to The Times that he was “in discussion” over selling it through Sotheby’s. Asked how he had felt when it was unearthed, he said: “The archaeological guys were more excited than me. I thought, ‘Oh my God, this will hold up the development’. At the end, the proof of the pudding is how much it is worth.”

An export licence will almost certainly be required for such an important piece, as it meets all the criteria governing the export of cultural objects.