Britain was turned into an island by an enormous flood that opened the Channel and changed the course of history, scientists have discovered.

Some time between 450,000 and 200,000 years ago, a natural land dam at the Strait of Dover failed, sending a wall of water surging into the once-dry basin that is now the Channel bed, with at least ten times the destructive power of the Indian Ocean tsunami, and redrawing the map of Europe.

Anything in the floodwater’s path would have been obliterated, as a discharge 100,000 times greater than that of the River Thames flowed through the breach.

It was one of the most severe floods known, according to research published in the journal Nature.

The legacy was the vast Channel river, which at its height drained half the waterways of Europe into the Atlantic, and cut Britain off from the rest of the Continent.

When the ice sheets that covered the British Isles and Scandinavia later melted, the entire basin was flooded to create the Channel as it is today.

Without this dramatic chain of events, which has been revealed by a new high-resolution sonar survey of the Channel floor, it is entirely possible that Britain would never have become an island or developed the seafaring culture that eventually turned much of the global map pink.

While scientists do not yet understand what triggered the megaflood. one possibility is that it was started by a small earthquake at a moment when summer meltwaters in what is now the North Sea were high.

Jenny Collier, of Imperial College, London, a leader of the study team, said: “This was the event that finally made us an island, and therefore gave us our unique historical development.

“We don’t actually know why the dam failed. It is possible that it was the pressure of rising water and that it would have happened anyway, but there are little earthquakes in that area — there was one recently in Kent — and it is a tantalising possibility that one triggered the flood.

“It might never have happened, and Britain would have gone through history always joined to Europe.”

One consequence was to cut Britain off from the European mainland even during periods of heavy glaciation when sea levels were low, making it much harder for early humans to settle what was previously a peninsula.

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