Archaeologists excavating one of the most important Roman sites in Britain have made an "extremely rare" find.

The team digging at part of the Roman fortress in Caerleon near Newport found what they believe is a legionnaire's ceremonial standard.

Dr Peter Guest said he thought the iron flag, broken into three pieces, was the first of its type found in the UK.

He also believed it was likely to have belonged to a high ranking commander who was "not to be tampered with".

A legionnaire would carry a long staff bearing the 45cm standard.

Dr Guest, of Cardiff University, said: "It's a very unusual find and there's not more than a dozen of them.

"I don't know of any of that type in Britain.

"There are a few at fortresses and forts around the Rhine and Danube, the frontiers of the Roman Empire."

The standard would probably have featured some type of decoration such as plumes, which indicated that the carrier was no ordinary soldier.

He would probably have been on special assignment, perhaps with the legion's commander or other high-ranking member of the Roman government in Britain.

The standard was found by a team working at the fortress to try to find out more about what could be a 2,000-year-old warehouse.

A large trench has been opened over the building, which is thought to have supplied the Roman legion.

The building was discovered during surveys and trial excavations last year.

Caerleon was the main administrative centre for the Roman army in Wales by 74 AD and is one of the most important Roman sites in Britain.