Hundreds of fragile archaeological sites across Britain are being routinely plundered by thieves with metal detectors, according to a report published by English Heritage.
Such “nighthawkers” are rarely caught, however, while those who are prosecuted are often let off with little more than the legal equivalent of a smack on the wrist.
Although most of their finds are worthless, their removal robs the nation of the story they could tell, the report by Oxford Archaeology warns.
Nighthawkers search for antiquities using metal detectors before digging them out to steal.
They are increasingly turning to the internet to trade their goods on eBay, the auction website, and exchange information on vulnerable sites using online forums.
While the internet has made it easier for them, prosecutions remain at an all-time low, English Heritage claims.
There is no distinct offence for the illegal practice. Nighthawkers who are caught are usually prosecuted for theft and criminal damage, which means that the true extent of the problem remains unknown.
The survey found that 240 raids were reported to police between 1995 and 2008, 88 on Scheduled Monuments.
Norfolk (23), Essex (14), Oxfordshire (13), Suffolk (12), Lincolnshire and Kent (both 11) recorded the highest number of raids.
Researchers calculated that about one in 20 archaeological excavation sites was targeted by thieves. Roman sites were repeatedly targeted.
But the survey found only one in seven landowners who discovered they had been raided reported the crime.
Many did not bother because they thought the police and Crown Prosecution Service would not push for legal action.
The survey concluded there was a “vicious circle” in which “an apparent lack of response from the police” led to under-reporting.
John Browning, whose farm at Icklingham in Suffolk contains the remains of a Roman villa, said he helped catch some nighthawkers “red-handed” on his farm, with Roman coins in their pockets.
But he said: “They were just bound over for 12 months and made to pay £38 costs. They were not even fined.”
He added: “When nighthawkers are fined it’s less than overstaying on a parking meter.
“The courts equate them with those guys on the beach looking for small change.”
In reality they sometimes travelled hundreds of miles to raid a site. He had encountered nighthawkers armed with knives and chains, he said.
Thousands of holes had been dug on his property over the years, he recalled, with raids sometimes happening twice a week.
Sir Barry Cunliffe, the chairman of English Heritage, is calling for a national database of nighthawking incidents to be established.
He said: “Nighthawkers, by hoarding the finds or selling them on without recording or provenance, are thieves of valuable archaeological knowledge that belongs to us all.
“Even in the case when the finds are retrieved, the context of how and where exactly the finds were found has been lost, significantly diminishing their historical value.
“In the cases of internationally important material the loss of the unique evidence that these objects provide on our common history and origins is especially poignant.
“By painting a clearer picture of the crime, this survey will help us to combat it more effectively.”
The survey highlighted big differences in the ways police forces combated the problem, with Kent singled out for praise.
A Suffolk Police spokesman said officers worked “closely” with Mr Browning on the problem, once despatching a helicopter and dogs to track suspects.
“We take these offences seriously,” he said.