Paint-wielding vandals may have caused irreparable damage to an ancient stone circle in what is thought to be an April Fools Day prank.
More than 70 stones have been daubed with paint
The Rollright Stones on the Oxfordshire/Warwickshire border have been daubed in yellow gloss paint.
It could cost tens of thousands of pounds to clean up the Neolithic circle, which is thought to be even older than Stonehenge.
The damage done to more than 70 stones was discovered on Thursday.
The circle, which could date back to as early as 2500 BC, also boasts important examples of lichen dating back hundreds of years.
Conservation experts from English Heritage, who helped restore stones at Avebury, have been brought in to assess the damage.
It is feared that some of the paint will have to be left on to protect the lichen from being damaged by any cleaning process.
Site manager Dohn Prout told BBC News Online: "It could not be anyone who loves the stone circle in any way, shape or form.
"It cannot be a religious group because they have not painted any religious symbols.
"It cannot be anyone with a grudge - we have not upset anyone.
"I suppose it is an April Fool's prank that has gone incredibly wrong."
Thames Valley Police are currently investigating the attack which is thought to have happened between 1900 BST on Wednesday and 0945 BST on Thursday.