Archaelogists have warned that Britain's historical artefacts are being looted by unscrupulous metal detector enthusiasts.
They believe "nighthawking" - illegal searches at protected historical sites, the removal of items without permission from landowners or the failure to declare valuable finds - is being fuelled by the internet, which allows criminals to make thousands of pounds selling jewellery and ancient coins.
The level of new housing developments in the south-east could also be fuelling the practice by providing a rising number of potentially lucrative locations where topsoil has already been removed.
English Heritage and the British Museum are so alarmed they have commissioned a £100,000 study into the practice. It could lead to new legislation to combat offenders.
Roger Bland, of the British Museum, said: "We know it's going on because we find the evidence - holes dug in the middle of fields."
There are an estimated 30,000 metal detecting enthusiasts in Britain. Peter Welch, founder of Weekend Wanderers Metal Detecting UK, said that most were law abiding.
But he added: "There's no doubt there are nighthawkers about, although I would have thought any further legislation is only likely to impact on the law-abiding. What is needed is to inform people about the laws."
This summer is the 10th anniversary of the introduction of the Treasure Act, which forces detectors to declare coins or gold or silver trinkets more than 300-years-old.
The finder has to hand over the item for a price agreed by a group of experts, although they must split the fee with the landlord on whose property it was found.
Of the items which were legally declared by metal detectors last year, the most expensive was a £350,000 gold necklace found near Newark.
A spokesman for Oxford Archaeology, the company which is carrying out the research, said they had received reports of nighthawkers threatening legal metal detector enthusiasts.
"People get abusive phone calls in the middle of the night and that sort of thing," she said. "We want to raise its profile as a criminal activity."
