English Heritage has asked members of the public to help stop a rise in illegal metal detecting at its ancient sites.
The field used in the Battle of Hastings in 1066 is among the locations, including medieval castles and Roman forts, which have been targeted by illegal detecting, known as “nighthawking”.
The conservation charity said such action was illegal and “robbing us of our past”.
It said that December was the worst month for reported incidents in more than four years, with double the number of metal detecting activities recorded in 2019 than in 2017.
Chief executive Kate Mavor said the charity was already working with police but needed the public’s help to report any suspicious activity.
“Illegal metal detecting robs us of our past,” she said.
“Once items are spirited away they can never be replaced, and the evidence of those who went before us is lost forever.
“We are working closely with Historic England and local police services to combat this form of heritage crime.
“We are now asking the wider community to act as our ‘eyes and ears’ and to report any suspicious activity on or near English Heritage sites to the police.”
Mark Harrison, head of crime strategy at Historic England, said: “Illegal metal detecting is not a victimless crime.
“We may never see or fully understand the objects taken or damaged because they have been removed from their original sites with no care or record as to their history or context.”
The site of the Battle of Hastings in East Sussex and Goodrich Castle in Herefordshire were amongst the sites that have been targeted by “nighthawks”.
Old Sarum in Wiltshire, the site of Salisbury’s original cathedral, has also been attacked multiple times, and Wiltshire Constabulary have appealed for witnesses.
Most of the more-than-400 sites in the care of English Heritage are unstaffed and free to enter. The charity is reviewing its security arrangements.
Metal detecting has been a hobby for many since detectors first became available on the mass market in the 1970s.
English Heritage said that in 2019 there were 12 recorded incidents of illegal metal detecting at its sites, with four sites alone targeted in December.
It said that “the sheer scale of the attacks, with up to 75 holes found to have been dug at each site” was of particular concern.