One of several Bronze Age axe heads taken illegally and sold on eBay
Britain's heritage is under threat from illegal metal detector users - or "nighthawkers" - who face little chance of being caught, a report says.
The UK-wide study found the threat to archaeological items was high, but prosecutions were at an all-time low and penalties "woefully insufficient".
The English Heritage-commissioned report said criminals were using auction websites to sell antiquities.
Many items are said to be worth little financially but of historical value.
Illegal metal detecting is defined as "the search and removal of antiquities from the ground using metal detectors without the permission of the landowners or on prohibited land such as scheduled monuments".
It is a form of theft and can be prosecuted under the Theft Act.
English Heritage said responsible metal detecting provided a valuable record of history, but nighthawkers - by hoarding the finds or selling them on without recording - were thieves.
The crime is most prevalent in central and eastern England but the survey found it was almost unheard of in Northern Ireland.
Counties with high incidences of nighthawking included Lincolnshire, Norfolk and Suffolk.
English Heritage said the 240 sites attacked between 1995 and 2008 were likely to be a fraction of the true scale of the under-reported crime.
Yorkshire (30 sites hit)
Source: English Heritage
More than a third of the attacks were at scheduled monuments - key sites of historical interest.
The study found only one in seven landowners who discovered they had been targeted by nighthawkers informed the authorities.
Only 26 cases resulted in legal action, with most offenders handed a small fine, in one case for as little as £38.
Researchers also found about one in every 20 archaeological excavation sites was targeted by thieves.
They said Roman sites often served as a honey-pot for thieves, and could be targeted repeatedly, particularly after the land had been ploughed.
Police say some thieves have formed loosely-connected networks to trade information, often in online forums, about new and vulnerable sites.
The axes are to go on display at the Buckinghamshire County Museum
Ch Insp Mark Harrison, from Kent Police, said farmers had been threatened after confronting groups of men trespassing on their land at night.
Sir Barry Cunliffe, English Heritage chairman, called for better guidance for police on the impact of nighthawking and a national database to detail the extent of the problem.
He said nighthawkers were "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," he said.
"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."
The report also recommended that antiquities sellers should be forced by law to prove the provenance of their goods and called on auction websites to monitor more closely items put up for sale more.