200 sites targeted ‘tip of iceberg’

But an English Heritage spokeswoman stressed it is “difficult to quantify” the true extent of the problem, and the study suggested the reported incidents were “only the tip of the iceberg”.

The National Council for Metal Detecting has established a code of conduct.

The council’s general secretary, Trevor Austin, who lives in Doncaster, claimed only a “tiny number” of the estimated 20,000 metal detectorists nationally are guilty of nighthawking. He said: “This is theft like any other theft, and it is something that any responsible metal detectorist would have nothing to do with whatsoever.”