A DNA study of Britons has shown that genetically there is not a unique Celtic group of people in the UK.
According to the data, those of Celtic ancestry in Scotland and Cornwall are more similar to the English than they are to other Celtic groups.
The study also describes distinct genetic differences across the UK, which reflect regional identities.
And it shows that the invading Anglo Saxons did not wipe out the Britons of 1,500 years ago, but mixed with them.
Published in the Journal Nature, the findings emerge from a detailed DNA analysis of 2,000 mostly middle-aged Caucasian people living across the UK.
The individuals included had all four of their grandparents living close to each other in a rural area.
This selection criterion enabled the researchers, led from Oxford University, to filter out 20th-Century immigration and to peer back to migration patterns more than 1,000 years ago.
According to Prof Peter Donnelly who co-led the study, the results show that although there is not a single Celtic group, there is a genetic basis for regional identities in the UK.
"Many of the genetic clusters we see in the west and north are similar to the tribal groupings and kingdoms around, and just after, the time of the Saxon invasion, suggesting these kingdoms maintained a regional identity for many years," he told BBC News.
Prof Donnelly and his colleagues compared genetic patterns now with the map of Britain in about AD 600, after the Anglo Saxons had arrived from what is now southern Denmark and Northern Germany. By then, they occupied much of central and southern England.
"We see striking similarities between the genetic patterns we see now and some of these regional identities and kingdoms we see in AD 600, and we think some of that may well be remnants of the groupings that existed then," he explained.
A map of different genetic groupings reveals subtle but distinct differences between those sampled in West Yorkshire and the rest of the country.
There is also a marked division between the people of Cornwall and Devon that almost exactly matches the county border. And the People of Devon are distinct again to those from neighbouring Dorset.
The Wellcome Trust-funded study, which is part of the People of the British Isles Research Project, also found that people in the north of England are genetically more similar to people in Scotland than they are to those in the south of England.
It also finds that people in North and South Wales are more different from each other than the English are from the Scots; and that there are two genetic groupings in Northern Ireland.
Prof Mark Robinson, an archaeologist who works with Prof Donnelly at Oxford University, said he was "very surprised" that Celtic groups in Cornwall, Wales, Northern Ireland and Scotland had such different genetic patterns.
"I had assumed at the very early stages of the project that there was going to be this uniform Celtic fringe extending from Cornwall through to Wales into Scotland. And this has very definitely not been the case," he told BBC News.
The researchers did see distinct genetic groups within those regions but those groups were quite different from each other, according to Prof Donnelly.
"Although people from Cornwall have a Celtic heritage, genetically they are much, much more similar to the people elsewhere in England than they are to the Welsh for example," said Prof Donnelly.
"People in South Wales are also quite different genetically to people in north Wales, who are both different in turn to the Scots. We did not find a single genetic group corresponding to the Celtic traditions in the western fringes of Britain."
Into the Dark Ages
The finding is the first genetic evidence to confirm what some archaeologists have long been arguing: that Celts represent a tradition or culture rather than a genetic or racial grouping.
Prof Robinson noted that the results also shed light on what happened during Britain's Dark Ages, in the years between AD 400 and AD 600, after the Romans left.
Towns were abandoned; the language over much of what became England changed (to Anglo Saxon, which became English); pottery styles altered; so too even the cereals that were grown, following the arrival of people from the base of the southwest Danish peninsula and northwestern Germany (the Anglo Saxons).
Some historians and archaeologists had wondered whether these changes occurred as a result of the Saxons entirely replacing the existing population as they moved westwards. That might have happened if the Saxons introduced disease, for example.
Others researchers suggested that the existing population simply dropped their old ways and adopted the Saxon way of life.
The new analysis shows a modest level of Saxon DNA, suggesting that the native British populations lived alongside each other and intermingled with the Anglo Saxons to become the English.
There is some evidence in the study that intermingling did not happen immediately following the Saxons' arrival, but occurred at least 100 years later. This suggests that Britons and Saxons had separate communities to begin with, and then over time they began to merge.
Northern Irish groupings
This may well be one of the first instances where genetics has been used to clear up historical controversy.
The study seems to confirm the view that Celts retained their identity in western and northern areas of England where the regions were incorporated into Anglo Saxon territory by conquest.
But what could account for the variation in the DNA of those of Celtic ancestry in Cornwall, Wales and Scotland? Time would be one possibility, according to Prof Donnelly.
"If groups have been separated for a period of time, they will diverge genetically so some of the differences we see genetically are the result of those kinds of effects," he said.
The study also notes that there are two genetic groupings in Northern Ireland: one of which also contains individuals across the sea in western Scotland and the Highlands; the other contains individuals in southern Scotland and southern England.
The former appears to reflect the kingdom of Dalriada 1,500 years ago; the other probably represents the settlers of the Ulster Plantations.
And in Orkney, the study finds clear evidence of Norwegian DNA, as might be expected from the Viking settlement of the Islands.
Interestingly, it persists at fairly low levels, suggesting that the Vikings and the existing populations coexisted and intermingled more than people had expected - in the way that occurred with the Anglo Saxons.
The Viking armies that laid waste to parts of England, and for a while ruled what became known as the Danelaw, left little if any genetic trace, confirming that their success was due to their military prowess rather than large-scale population movement.
Likewise, the Norman conquest of England did not leave any genetic evidence.
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