The UK has now started the formal process of leaving the EU, but scientists say they have evidence of a much earlier “Brexit”.
They have worked out how a thin strip of land that once connected ancient Britain to Europe was destroyed.
The researchers believe a large lake overflowed 450,000 years ago, damaging the land link, then a later flood fully opened the Dover Strait.
The scars of these events can be found on the seabed of the English Channel.
The study is published in the journal Nature Communications.
Professor Sanjeev Gupta, who led the study, from Imperial College London, said: “This was really one of the defining events for north west Europe – and certainly the defining event in Britain’s history.
“This chance geological event, if it hadn’t happened, would have meant Britain was always connected to the continent.”
More than half a million years ago, in the midst of an Ice Age, a land bridge connected Dover in the South of England to Calais in northern France.
Immediately to the north of it, was a huge glacial lake, which had formed at the edge of an ice sheet that covered much of Europe.
The researchers believe that this lake started to overflow, sending vast amounts of water crashing over the land bridge.
The evidence for this was found at the bottom of the English Channel.
Decades ago, engineers who were surveying the seabed for the Channel Tunnel, discovered a series of mysterious large underwater holes.
Now further scrutiny has revealed that they were most likely caused by the lake overspill.
Prof Gupta said: “These holes are now in-filled with sediment, but what’s interesting is that they are not linear features like canyons or valleys – they are isolated depressions.
“And they occur in a line – a whole series of them stretching between Dover and Calais. And they are huge, 100m-deep carved into the bedrock and hundreds of metres to several kilometres in diameter.
“So we interpret these as giant plunge pools. We think there was basically lake water plunging over this rock ridge in the Dover Strait through a whole series of waterfalls, which then eroded and carved out these depressions.
“It’s difficult to explain them by any other mechanism.”
The researchers believe the lake started to overflow about 450,000 years ago, which would have seriously weakened the land bridge.
But they think a second catastrophic flood that took place about 150,000 years ago would have destroyed it altogether.
“We see this huge valley carved through the strait, about eight to 10km wide… and it has a lot of features that are suggestive of flood erosion,” said Prof Gupta.
Co-author Jenny Collier, also from Imperial College London, said it was not clear what caused either of these events.
She said: “Perhaps part of the ice sheet broke off, collapsing into the lake, causing a surge that carved a path for the water to cascade off the chalk ridge.
“In terms of the catastrophic failure of the ridge, maybe an earth tremor, which is still characteristic of this region today, further weakened the ridge.
“This may have caused the chalk ridge to collapse, releasing the megaflood that we have found evidence for in our studies.”
The researchers would now like to work out more precise timings of the “geological Brexit”.
This would mean drilling into the bottom of the Dover Strait and analysing the age of the sediment.
“But that would be a huge undertaking,” admitted Prof Gupta.
“The English Channel is the world’s busiest shipping lane and it has huge tidal currents. It will be hugely challenging.”