A 5,000-year-old mystery has been sparked after part of a human skull was found lying on a river bank.

Archaeologists have said the unbroken piece of upper skull is in "fabulous" condition with the intricate marks from the blood vessels still visible on the inner surface.

The smooth dark outer side gives only a tantalising glimpse as to what the person may have looked like, although there are "tentative" suggestions it may have belonged to a woman in middle age living in the Neolithic period - around the time Stonehenge was built.

The skull is not only prompting questions about the person it belonged to, but where it may have come from according to the experts now studying the "exceptional find".

A dog walker first stumbled across the skull piece, which is about 15cms in length and 10cm in width, earlier this year but initially thought it was a ball or a coconut shell.

However, the next day the man went back to the site on the banks of the River Avon near Pershore in Worcestershire, England for a closer look and, realising what it was, called the police.

Detectives from West Mercia Police initially investigated but then contacted experts at Worcestershire Archaeology, who sent the skull to be radiocarbon dated.

"When I first saw the skull, I thought it may have been Anglo-Saxon or Roman but I knew that it was not recent due to the colour," said Nick Daffern, senior archaeologist.

"But we were all surprised when the radiocarbon dating put it at between 3,338 BC and 3,035 BC, or about the middle Neolithic period."

"It is so well preserved, it is unthinkable that this had been in the river for any length of time which begs the question as to where it has come from.

"We know of Roman, Saxon and medieval burials along the river, but this is very rare - it is an exceptional find.

"What it suggests is that we have a Neolithic burial site very near here - we just don't know where."

He added: "I don't think it was found where the remains were buried, I think we've got a riverside burial and then flooding has brought this down the river.

"Finding that burial site though would be like finding a needle in a haystack."

Mr Daffern said that without the rest of the skeleton it was difficult to draw conclusions about the person found, and certainly there is no clue as to how they met their death.

"Both myself and a forensic anthropologist believe it is a woman due to the slightness of the skull and the lack of any brow ridges although our conclusions are very tentative because we're dealing only with the top of a skull," he added.

"There's no trauma to the bone, and where it has broken those are natural breaks, nor is there any sign of disease so we've no idea as to cause of death.

"The natural fusion of the bone in the skull leads me to believe it may be an older woman, possibly in her 50s, but that is very tentative again.

"Unfortunately, it remains a bit of a mystery."

The find is a few miles from Bredon Hill, which has been a scene of human activity down the ages and still boasts the earthen ramparts of what was an Iron Age hill fort, however finds of Neolithic remains are rare.

"Whenever we come across Neolithic remains, there seems to be a solid dividing line between where they buried their dead, and where they lived and that is no accident," he said.

"But it is frustrating as an archaeologist because although we have the physical evidence, we still don't have the answers as to why."

The skull is only the second set of Neolithic remains to be found in the county, although two large 6,000-year-old 'halls of the dead' were found in nearby Herefordshire this year but without any human remains present.