The graffiti was discovered by staff on Tuesday

The man, who might speak with a Liverpudlian accent, is said to be 5ft 6 in, of medium build with fair hair.

He may be using the name Brian Finlay and make conversation about the Celts, police on Orkney added.

Staff found the graffiti daubed on the 5,000-year-old site - including the words "Scouse Celts" - on Tuesday.

Monument managers said vandals had entered one of the houses at the site and drawn a smiley face, their name and the date with black marker pen, defacing both the dresser and one of the bed posts.

Ruled out

It was initially thought that the vandalism may have been carried out by tourists travelling round the island in a silver people carrier.

However, police ruled the group out of their inquiries after questioning the vehicle's occupants earlier.

The man they are now looking for is believed to be sleeping rough on Orkney and has a backpack with him.

Historic Scotland, which is responsible for the upkeep of the world heritage site, said that it was liaising with its conservation centre in Edinburgh to find the best way of removing the graffiti as soon as possible.

"Graffiti removal on buildings of historical or architectural importance needs to be handled sensitively," said a spokeswoman.

Our visitors have reacted with surprise and sympathy to the damage caused

Historic Scotland spokeswoman

"The careful process needs to ensure any long-term damage caused by the graffiti is kept to a minimum and the stone itself is not further damaged as part of the graffiti removal process.

"Our visitors have reacted with surprise and sympathy to the damage caused."

The cost - which is expected to be substantial - is not yet known.

Skara Brae was first discovered following a major storm in 1850, which battered the island to such an extent that an outline of a series of stone buildings was revealed.

A series of excavations was carried out over the next century, finally revealing eight stone dwellings which were originally believed to be an Iron Age settlement.

However, in the 1970s radiocarbon dating showed that the village was originally built between 3200 and 2200 BC.