Don't tell the locals, but the hordes of British holidaymakers who visited Spain this summer were, in fact, returning to their ancestral home.

A team from Oxford University has discovered that the Celts, Britain's indigenous people, are descended from a tribe of Iberian fishermen who crossed the Bay of Biscay 6,000 years ago. DNA analysis reveals they have an almost identical genetic "fingerprint" to the inhabitants of coastal regions of Spain, whose own ancestors migrated north between 4,000 and 5,000BC.

The discovery, by Bryan Sykes, professor of human genetics at Oxford University, will herald a change in scientific understanding of Britishness.

People of Celtic ancestry were thought to have descended from tribes of central Europe. Professor Sykes, who is soon to publish the first DNA map of the British Isles, said: "About 6,000 years ago Iberians developed ocean-going boats that enabled them to push up the Channel. Before they arrived, there were some human inhabitants of Britain but only a few thousand in number. These people were later subsumed into a larger Celtic tribe... The majority of people in the British Isles are actually descended from the Spanish."

Professor Sykes spent five years taking DNA samples from 10,000 volunteers in Britain and Ireland, in an effort to produce a map of our genetic roots.

Research on their "Y" chromosome, which subjects inherit from their fathers, revealed that all but a tiny percentage of the volunteers were originally descended from one of six clans who arrived in the UK in several waves of immigration prior to the Norman conquest.

The most common genetic fingerprint belongs to the Celtic clan, which Professor Sykes has called "Oisin". After that, the next most widespread originally belonged to tribes of Danish and Norse Vikings. Small numbers of today's Britons are also descended from north African, Middle Eastern and Roman clans.

These DNA "fingerprints" have enabled Professor Sykes to create the first genetic maps of the British Isles, which are analysed in Blood of the Isles, a book published this week. The maps show that Celts are most dominant in areas of Ireland, Scotland and Wales. But, contrary to popular myth, the Celtic clan is also strongly represented elsewhere in the British Isles.