The Welsh and Irish Celts have been found to be the genetic blood-brothers of Basques, scientists have revealed.
The gene patterns of the three races passed down through the male line are all "strikingly similar", researchers concluded.
Ethnic links: Many races share common bonds
Basques can trace their roots back to the Stone Age and are one of Europe's most distinct people, fiercely proud of their ancestry and traditions.The research adds to previous studies which have suggested a possible link between the Celts and Basques, dating back tens of thousands of years.
"The project started with our trying to assess whether the Vikings made an important genetic contribution to the population of Orkney," Professor David Goldstein of University College London (UCL) told BBC News.
He and his colleagues looked at Y-chromosomes, passed from father to son, of Celtic and Norwegian populations. They found them to be quite different.
"But we also noticed that there's something quite striking about the Celtic populations, and that is that there's not a lot of genetic variation on the Y-chromosome," he said.
To try to work out where the Celtic population originally came from, the team from UCL, the University of Oxford and the University of California at Davis also looked at Basques.
"On the Y-chromosome the Celtic populations turn out to be statistically indistinguishable from the Basques," Professor Goldstein said.
The comparison was made because Basques are thought by most experts to be very similar to the people who lived in Europe before the advent of farming.
"We conclude that both of these populations are reflecting pre-farming Europe," he said.
Professor Goldstein's team looked at the genetic profiles of 88 individuals from Anglesey, North Wales, 146 from Ireland with Irish Gaelic surnames, and 50 Basques.
"We know of no other study that provides direct evidence of a close relationship in the paternal heritage of the Basque- and the Celtic-speaking populations of Britain," the team write in the journal Proceedings of the National Academy of Sciences.
But it is still unclear whether the link is specific to the Celts and the Basques, or whether they are both simply the closest surviving relatives of the early population of Europe.
What is clear is that the Neolithic Celts took women from outside their community. When the scientists looked at female genetic patterns as well, they found evidence of genetic material from northern Europe.
This influence helped even out some of the genetic differences between the Celts and their Northern European neighbours.
The work was carried out in connection with a BBC television programme on the Vikings.