Visitors to a 6,000-year-old site who are removing stones and piling them up to be "artistic" could be causing significant damage, experts say.
Stones from Stowe's Pound on Bodmin Moor, Cornwall, are being used to build the "fairy stacks" by people "probably unaware" they are breaking the law.
The stacks have been described as "historic vandalism".
The practice at the Scheduled Ancient Monument site has also been condemned by Historic England.
Stuart Dow and Roy Goutte, from Time Keepers, an amateur archaeology group, have called for signs to be erected at the site explaining its significance and warning that removing the stones "could see them end up in jail".
"They may think they're being artistic, but they don't realise the damage they're doing," Mr Dow said.
"Some of these may be lovely to look at, but knowing what they represent, I believe it's disrespectful to our ancestors."
The group says simple signage should prevent any more damage and in the meantime volunteers have offered to remove the stacks and put the stones back where they should be.
What is Stowe's Pound?
- One of a number of "tor enclosures" - circular stone walls near natural rock outcrops, especially tors, on hilltops or the sides of hills - found in Cornwall, dating back to 4000-3500BC
- It is believed Stowe's Pound, along with the other tor enclosures, were important sites for gathering and exchanging goods and ideas
- Two stone built ramparts survive at the site.
- They have the remains of house platforms and cairns, a mound of rough stones, inside them.
- Historic England says moving these stones is "eroding nationally protected archaeological features of the site"
"We don't think for a moment this is malicious, nevertheless it's historic vandalism and we have to try to stop it," Mr Goutte said.
Daniel Ratcliffe, inspector of Ancient Monuments at Historic England South West, said Stowe's Pound dates back to the Early Neolithic period - 4000 to 3500BC - and while moving the stones to build the "inevitably temporary" fairy castles may seem benign, it is actually eroding nationally-protected archaeological features.
"Taking away the stones is like rubbing out history," Mr Ratcliffe said.
He believes making people aware of the importance of the site was the answer to the problem, adding that Historic England would be willing to discuss possible options with landowner, the Duchy of Cornwall.
The site is protected by law under the Ancient Monuments and Archaeological Areas Act of 1979 and anyone damaging or removing objects could face criminal prosecution.