Historians are counting the cost after vandals used a stolen digger to damage an ancient monument in Bedfordshire.

Holes have been scraped inside the Iron Age Maiden Bower Fort at Sewell, near Dunstable, to make ramps for off-road motorbikes.

Earthworks which surround much of the fort have been damaged in two places.

The digger has also been used to scrape holes inside the ancient monument.

Whoever did it must be local enough to know that this is an ancient monument and that they weren't just destroying an old bank of earth David Warren, Manshead Archaeological Society

The vandals are thought to have struck twice. The first attack was some time over the Christmas break.

Now English Heritage has been informed by the police of more damage.

Dave Warren, site director with the local Manshead Archaeological Society, said he was disgusted.

"Whoever did it must be local enough to know that this is an ancient monument and that they weren't just destroying an old bank of earth," he said.

"They must have known exactly what they were damaging.

"They must have been hoping there might be something inside the fort.

"They weren't in ignorance of what they were damaging."

Substantial fine

Mr Warren said the monument had been eroded over time but had reached a more or less stable state.

The damage to the ramparts could be "the tip of the iceberg", he said.

History experts are still assessing how much damage has been done inside.

The fort is a registered national monument and anyone causing damage to it could face legal proceedings and a substantial fine if found guilty.