A world-renowned stone circle in Orkney, which is more than 4,000 years old, has been vandalised.

Damage to the Ring of Brodgar includes graffiti that has been engraved into one of the stones at the Neolithic site near Stenness. It is believed to have been caused sometime between Friday afternoon and Sunday morning.

Insp David Hall from Police Scotland said: “The stones at the Ring of Brodgar are priceless historical artefacts and the damage caused cannot simply be estimated in monetary terms.

“For someone to damage them in this way is a particularly mindless act. I would urge anyone who has visited the area over the last weekend to think back and if they believe they may have seen something suspicious, even if it didn’t seem of much note at the time, to let us know.

“We would also urge the public to continue to be vigilant at this site and report anything which could be of interest to police immediately.”

The ring originally comprised 60 stones, of which 36 survive.

It is within the Heart of Neolithic Orkney Unesco world heritage site, which also includes a large chambered tomb called Maeshowe, the Stones of Stenness and the Skara Brae settlement.

The ring was built around 2,500-2,000BC and covers an area of almost 8,500 sq metres (91,500 sq ft).

It is the third largest stone circle in the British Isles, behind Avebury and Stanton Drew, and the largest in Scotland.

One of the stones had previously been vandalised, carrying a Norse runic inscription, while at least one other has been struck by lightning.

Celebrated visitors include Sir Walter Scott, who in 1814 wrote of the ring and the Stones of Stenness that “Stonehenge excels these monuments, but I fancy they are otherwise unparalleled in Britain”.

Historic Environment Scotland notes suggestions that the sites and monuments in the surrounding area were used for astrological observations from the ring, but it is difficult to find conclusive evidence.

A spokeswoman for the public body said: “We were recently made aware of an incident of vandalism to the Ring of Brodgar. We would ask the public to be aware that causing reckless or deliberate damage to a scheduled monument is a criminal offence, and ask that anyone witnessing such acts report them to Police Scotland.”