Arsonists have attacked the ancient Men an Tol and Lanyon Quoit stones in Cornwall, and threatened to "reduce them to rubble" in protest over unspecified mistreatment of them in the past.
The stones, massive lumps of granite which have stood in isolated moorland for the past 5,000 years, are badly scorched and coated with a thick sticky mess, which the arsonists claimed was "stolen napalm".
The Men an Tol - the 'stone with a hole' - is one of the best known and most picturesque of all the thousands of west country stone age monuments.
A photograph showing fire blazing around the stones, and a letter claiming credit for the attack, were sent to the Cornishman newspaper, which has passed both to Devon and Cornwall police. The letter claims three people, calling themselves Friends of the Stone, carried out the attack "on the sacred night of November 5", and described it as "an attempt to make them better, or at least more aesthetically pleasing". There is an explicit threat of further attacks on both stone groups. "You do not deserve the heritage these monuments hold and therefore we intend to act further."
Some of the stones, which date from 3500 to 2500 BC, were re-erected and possibly rearranged by Victorian antiquarians - which may be the origin of the accusation in the letter that Lanyon Quoit is "a fake prophet" and that the Men an Tol was not correctly aligned. A police spokesman said forensic tests are being carried out at the stones, and on the letter and photograph. He added that the county's wealth of field monuments, many far from roads or buildings, are impossible to protect.
Archaeologists are aghast at the attacks, which follow a disturbing sequence of vandalism at other ancient sites. Three years ago some of Wiltshire's famous white horse chalk carvings were daubed with paint, and stones at Avebury were daubed with paint 18 months ago and again this summer.
A group calling itself `Cuckoo' claimed the last attack as a work of art, and in an email to the Guardian warned: "In years to come and as cuckooism spreads, I'm certain that your fellow hacks and art correspondents will be writing much more about exhibitions that have sprung up overnight on public and private property."
At the Cornish archaeology unit yesterday, Steve Hartgroves said: "I am at a loss to know what we can do. This is a new phenomenon. These monuments have been protected in the past by a respect, or suspicion, or dread - a general feeling that it is not a good idea to mess with them. That is now gone."
His unit helped mount guard on the stones over the period of the solstice last summer, when it was feared they would be sites for illegal rave gatherings, but that passed off without incident. The unit has been working recently with self-styled pagan groups, whose behaviour he praised as "highly responsible - they come, they hold their ceremonies, they take everything away with them, and they check next day that there is no rubbish lying about."
He said there had been a recent increase in damage to monuments. "We find people have been digging up the ground in front of the stones, and leaving little offerings, crystals, plastic beads, even 2p coins. It might seem harmless, and it is clearly intended as a sort of reverence, but it will make the stone circles archaeologically worthless when all the ground around them has been disturbed."
The Lanyon Quoit and Men an Tol are in the guardianship of English Heritage but stand on National Trust land. English Heritage, whose experts were examining the damage yesterday, issued a statement appealing to local people to report anything suspicious.
The trust's countryside officer, Jon Brookes, said: "This is craziness. I am appealing to these people: if you have got a problem, come to me and talk about it and let's see if we can work something out."