At one time, chisels would be handed to people visiting Stonehenge, so they could chip away at the ancient monument to get their own souvenirs.

But the practice has been outlawed since 1900, when landowner Sir Edmund Antrobus decided the site needed protecting and introduced charges.

Before then, anyone who visited the site could walk freely among the ancient stones.

Now, the stones are fenced off, with private access allowed only by special arrangement.

English Heritage said an attack on the revered stones on Thursday, during which a piece of the Heel Stone was chipped off with a hammer and screwdriver, was believed to be the first of its kind in many years.

Nearly 1m visitors a year flock to the 5,000-year-old World Heritage Site.