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The point when Britain became an island is a defining moment in our history.
Scientists have, however, always been puzzled as to exactly how we lost our land link with the Continent.
Now researchers say they had found the answer in the shape of a massive flood which breached the ridge joining Kent and France.
They said it would have taken only days for what we now know as the English Channel to have filled with water.
The event is thought to have taken place between 200,000 and 400,000 years ago. Britain's fate as an island was sealed by a second massive flood a few thousand years later, the team from Imperial College London said.
The flood waters came from the southern reaches of the North Sea which had been dammed off into a lake by the landbridge at Dover and glaciers to the north.
Once released, the water carved out a channel 15 miles wide and 50 yards deep through the chalky soil.
'We don't know why, but the ridge was breached, triggering what is likely to be the biggest flood ever,' said Dr Jenny Collier, a co-author of the study which examined the loss of the land link.
'It could have been caused by rising water in the lake, or even by a small earthquake.'
Her team took high-resolution sonar images of the valley formed by the first flood showing it was unusually straight and wide with erosion marks. They said this evidence suggested the valley - now under the Channel - was the result of a sudden flood, delivering one million cubic metres of water every second.
The so- called 'megaflood' may have lasted several months and by the time it subsided, a vast, uncrossable waterway separated Britain from the rest of Europe. The river was the largest in Europe, draining the Rhine, the Thames, the Solent, the Rother, the Seine and the Somme. It also created a barrier to Neanderthal hunters who had previously been regular visitors to Britain. Dr Sanjeev Gupta, a co-author of the study, said: 'This prehistoric event rewrites the history of how the UK became an island and may explain why early human occupation of Britain came to an abrupt halt for almost 120 000 years.' The second flood breached a dam that had formed around a ridge of rock north of Dover. This event sent another huge torrent of water into the English Channel and further widened the Straits of Dover.
Legend:
1 - continental (a) and maritime (b) glaciers, 2 - open sea, 3 - lakes, 4 - elongated
elevations, 5 - courses of waterways, 6 - primeval valleys (Grosvald 1983: 96-
97) and major sites of prehistoric art before (A) and after (B, C) the glacial
maximum.
Map of the maximum extent of the ice during the last ice age, around 20 000 years ago. This is by far the clearest version of the extent of the ice in the last ice ages I have ever seen.
Note in particular the extensive lakes ponded behind the ice, fed by the north flowing rivers.
Note also the increased size of the Caspian and Aral Seas, and the reduced size of the Black Sea.
Map of the extent of the ice during the last ice age 13000 years before the present.
Map of the extent of the ice during the last ice age 10500 years before the present, just before the final retreat of the ice.