Organised crime is being blamed for a rise in illegal metal-detecting at heritage sites, including one of England’s finest medieval castles and the battlefield of Hastings.

English Heritage said December last year was the worst month for such incidents in more than four years and there were more than double the number of incidents in 2019 as there were in 2017.

In some cases there were up to 100 holes where the illegal metal detectorists – known as nighthawkers – had dug up the soil. “How many of those are productive we just don’t know,” said Win Scutt, a properties curator and archaeologist at English Heritage.

The organisation is calling on members of the public to become its “eyes and ears” and report suspicious activity to the police. However, after some violent incidents it advised people not to confront nighthawkers.

Scutt said it was not only about artefacts being dug up, taken and presumably sold. “It is the damage to the sites. They are destroying the archaeological record. You can compare it to a scene of crime and seeing a knife and a gun and thinking: ‘I’ll be helpful, I’ll take them along to the police,’ and they say: ‘You’ve just destroyed all the information.’

“This is why we protect our sites so carefully. Not even we dig them. We are trying to preserve the record for future generations.”

Scutt said the activity was distressing. “These sites are in a sense libraries of archaeological information and poking holes in them is like burning down a library of documents that have never been read.”

English Heritage said sites targeted included the Hastings battlefield in East Sussex, Goodrich Castle in Herefordshire and Old Sarum in Wiltshire, the site of Salisbury’s original cathedral.

Facebook Twitter Pinterest Evidence of nighthawking at Old Sarum. Photograph: English Heritage

The majority of the more than 400 places maintained by English Heritage were unstaffed and free to enter.

Scutt said he did not think ordinary metal detectorists would be naive enough to think it was acceptable or legal to use their detectors on scheduled monuments. “There seems to be a criminal element deliberately going on to our sites. We attribute the increase to organised crime … they are going for the goodies and they don’t care.”

Mark Harrison, the head of crime strategy at Historic England, said it was 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.”