Archaeologists excavating the area around an historic stone formation believe a man-made ring of stones, known as a stone circle, may have been found at the site.

Arthur’s Stone in Dorstone, Herefordshire, is said to mark the spot where King Arthur slew a giant.

Professor of archaeology at The University of Manchester Julian Thomas said a survey had shown up a circle about 20m (65.6ft) across that seemed to “not be of natural origin”.

Tours around Arthur’s Stone will take place over the coming weeks for people wanting to see this year’s excavations.

Experts from English Heritage and the University of Manchester have previously said Arthur’s Stone was one of the country’s most significant prehistoric monuments.

Slabs set in the hills above Herefordshire’s Golden Valley form a 5,000-year-old Neolithic burial chamber, formed by nine upright stones with a capstone on top.

It has informed legends, with indentations on its surface said to have been made as the giant fell backwards after being slain by King Arthur.

Prof Thomas said when work started at Arthur’s Stone, a trench was opened up “the other side of the hedge from the monument”.

He added: “[We] found that there are further prehistoric features there, so we’ve got an avenue which starts out as an avenue of timbers and is replaced by an avenue of stones.

“[This] runs down the hill from the monument itself and as a response to that, we conducted further surveys.”

Prof Thomas said one thing it showed up was a circle, which “looks like it’s perhaps a barrow or a timber circle or a stone circle”.

But it was “certainly something made by human beings” and about half of that was now being opened up, with a stone circle thought to be the most likely outcome.

There were plenty of stone circles in Scotland and Wales, but “not so many” in Herefordshire and the West Midlands, he said, “so it would be a really exciting find if we’ve got one here”.