SCIENTISTS have found that Britain owes its island status to a catastrophic flood that in less than 24 hours swept away the hills that once joined the country to France.

The flood, which took place between 400,000 and 200,000 years ago, instantly turned Britain from being a peninsula of continental Europe into a separate entity, changing for ever the way it would develop.

The finding has emerged from an advanced sonar survey of the bed of the Channel that revealed huge scour marks, deep bowls and piles of rock that could have been created only by a giant torrent of water. If confirmed it will force a revision of British prehistory.

It had been thought that the Channel had formed by slow erosion combined with rises in sea level that took place over millions of years, rather than by a sudden biblical-style catastrophe.

“This could have been one of the most powerful flood events ever known on earth,” said Professor Chris Stringer, head of human origins at the Natural History Museum in London. “It would have cut through the chalk hills joining Britain to Europe.”

Stringer’s colleague Sanjeev Gupta, from Imperial College, London, used advanced sonar to survey the sea floor several miles off the Sussex coast.

The team was surprised to find the remains of a huge valley, running southwest from the Strait of Dover.

“In places this valley is more than seven miles wide and 170ft deep, with vertical sides. Its nearest geological parallels are found not on earth but in the monumental flood terrains of the planet Mars,” said Gupta in an abstract published at an academic conference.

“This suggests the valley was created by catastrophic flood flows following the breaching of the Dover Strait and the sudden release of water from a giant lake to the north.”

In the scenario envisaged by Stringer and Gupta, there was a high chalk ridge linking Britain and France running roughly between Dover and Calais. Northeast of this ridge the land sloped down until it met the North Sea.

During one of Europe’s glaciations, an ice cap up to a mile thick reached so far south that it stretched from Scotland to Denmark, effectively damming the North Sea. This turned it into a freshwater lake which, fed by rivers, deepened over thousands of years.

“The lake would have been hundreds of feet above the then sea level,” said Stringer, whose new book Homo Britannicus: The Incredible Story of Human Life in Britain is published next week. “One day it just overflowed the top of the chalk ridge and started pouring over. Once the torrent started it would have ripped through the soft chalk and poured down towards the Atlantic.”

Global sea levels were far lower then than now because so much water was locked up in the ice caps. This meant that Britain was joined to France all the way along its south coast — and the cascading water had to carve its way across the landscape.

A group of French researchers, working separately from Gupta, has traced the course of that flood, finding a giant submerged river valley running southwest along the seabed between Normandy and Cornwall.

The discovery of the Channel flood may help to solve one of the enduring mysteries of British archeology — the apparent abandonment of the British Isles by humans for 120,000 years.

Researchers like Stringer have found that early humans first arrived in Britain at least 700,000 years ago. They were driven out by repeated glaciations but kept on returning whenever the land warmed up. However, between 180,000 and 60,000 years ago they vanished completely.

During that time Britain had periods of Mediterranean warmth when wildlife abounded. Early humans would have thrived — but there are no signs of them.

Nick Ashton, a senior curator at the British Museum, said that the flood and the resulting giant river valley could be the explanation.

“This giant river would have been a complete barrier to the Neanderthal peoples found in Europe at that time,” he said.

Britain might have been the promised land — but people could not get in.

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