Suspected souvenir hunters broke into Stonehenge and vandalised the ancient monument, English Heritage said today.

Two men used a hammer and screwdriver to chip away at the Heel Stone - a 16ft (4.8m) megalith at the 5,000-year-old site - before they were spotted by security guards and chased away.

The vandals managed to hack off a small 10p-sized piece from the sarsen stone. The two men got away in a red Rover 400.

The incident is thought to be the first act of vandalism at the world-renowned site for decades.

A spokeswoman for English Heritage said: "Thanks to the vigilance and quick action of the security team at Stonehenge, very minimal damage was caused to the Heel Stone at Stonehenge.

"A tiny chip was taken from the north side of the Heel Stone with a screwdriver and hammer, but as soon as the two men were spotted by security guards they escaped over the fence and drove off.

"This is now a matter for the police."

English Heritage said the men may have been after a souvenir from the prehistoric landmark. This was once a legitimate practice and guides would even hand out chisels to visitors.

The raiders struck at around 10pm on May 15 but news of the offence has only just been released.

Police said it is believed the men could be the same two people caught on CCTV acting suspiciously a few days earlier.

A spokeswoman for Wiltshire police said: "Two male offenders were seen disturbing the monument with a hammer and screwdriver.

"A two-and-a-half-inch line was left on the stone. It is believed they could be two men seen acting suspiciously on a previous occasion.

"They left the site in a red Rover, with plastic sheeting covering the driver's side window."

Archaeologists have been excavating the ancient site to discover more about its mysterious origins. The dig is the first to take place at Stonehenge for more than 40 years.