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.

Download the new Independent Premium app Sharing the full story, not just the headlines

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.

"Although Celtic countries have previously thought of themselves as being genetically different from the English, this is emphatically not the case," Professor Sykes said.

"This is significant, because the idea of a separate Celtic race is deeply ingrained in our political structure, and has historically been very divisive. Culturally, the view of a separate race holds water. But from a genetic point of view, Britain is emphatically not a divided nation."

Daily coronavirus briefing No hype, just the advice and analysis you need Enter your email address Continue Continue Please enter an email address Email address is invalid Fill out this field Email address is invalid Email already exists. Log in to update your newsletter preferences Register with your social account or click here to log in I would like to receive morning headlines Monday - Friday plus breaking news alerts by email Update newsletter preferences

Origins of Britons

Oisin

Descended from Iberian fishermen who migrated to Britain between 4,000 and 5,000BC and now considered the UK's indigenous inhabitants.

Wodan

Second most common clan arrived from Denmark during Viking invasions in the 9th century.

Sigurd

Descended from Viking invaders who settled in the British Isles from AD 793. One of the most common clans in the Shetland Isles, and areas of north and west Scotland.

Eshu

The wave of Oisin immigration was joined by the Eshu clan, which has roots in Africa. Eshu descendants are primarily found in coastal areas.

Re

A second wave of arrivals which came from the Middle East. The Re were farmers who spread westwards across Europe.

Roman

Although the Romans ruled from AD 43 until 410, they left a tiny genetic footprint. For the first 200 years occupying forces were forbidden from marrying locally.