The team, led by the University of Southampton, say the arena could have held up to 2,000 people and been used for gladiator games or animal baiting.

It was found inside a gigantic imperial-style palace within the well-preserved old harbour of Portus.

Experts said the entire site deserved greater recognition.

The excavation team, which also included archaeologists from Cambridge University, has spent two years at Portus, about 20 miles (32km) from the Italian capital.