Britain's archaeological heritage is being plundered by illegal metal detector users who face little danger of being caught, a report said today.
The first comprehensive national survey of its kind revealed thieves armed with state-of-the-art equipment are raiding some of the nation's most sensitive heritage sites.
Researchers found knowledgeable criminals, dubbed nighthawks, are using auction websites such as eBay to cash in on what was once an illicit hobby.
Police said some thieves have formed loosely-connected networks who trade information, often in online forums, about new and vulnerable sites.
One senior Kent officer said there have been cases of farmers being threatened after confronting groups of men trespassing on their land at night.
English Heritage, who commissioned the study, said many stolen items are worth very little, but their valuable historical context is lost for ever.
But although the threat of nighthawking remains high, experts said the chances of prosecution remain at an all time low and penalties are "woefully insufficient".
Sir Barry Cunliffe, English Heritage chairman, called for better guidance for police and a national database to accurately portray the extent of the problem.
He said: "Responsible metal detecting provides a valuable record of history, but illegal activities bring responsible ones into disrepute.
"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."
Nighthawking is the search and removal of antiquities from the ground using metal detectors without the permission of landowners or where the practice is banned.
The problem emerged in the early 1970s as metal detecting first became a popular hobby and has become increasingly prevalent.
English Heritage said 240 police reports of raids between 1995 and 2008 are likely to be just a fraction of the true scale of the under-reported crime.
The study found only one in seven landowners who discovered they had been targeted by illegal metal detector users informed the authorities.
Rural counties such as Norfolk (23), Essex (14), Oxfordshire (13), Suffolk (12), Lincolnshire and Kent (both 11) recorded the highest number of sites hit by nighthawking.
Researchers also found about one in every 20 archaeological excavation sites are targeted by thieves.
Roman sites often serve as a honeypot for thieves and can be targeted repeatedly, particularly after the land has been ploughed.
More than a third of sites attacked by illegal metal detectorists (88) were scheduled monuments, key sites of historical interest.
Only 26 cases resulted in legal action, with most offenders handed a small fine which in one case was just £38.
Dr Pete Wilson, head of roman archaeology at English Heritage, said better guidance is required for police and prosecutors.
He said this should outline the best ways of collecting evidence and lead to more successful prosecutions.
Dr Wilson said antiquities sellers should be forced by law to prove the provenance of their goods and called on auction websites to monitor items put up for sale more closely.