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Barmy Boris Johnson wanted to build a Brexit road tunnel linking Britain and France under the English Channel, it has emerged.

The Foreign Secretary plotted the multibillion-pound, submarine highway in a bonkers bid to show the EU the UK was not giving up on Europe despite quitting the bloc.

Mr Johnson, who led the Leave campaign and previously wanted to build a huge airport in the Thames estuary, only abandoned the idea after being talked out of it by long-suffering aide Will Walden, according to a new book serialised in the Sunday Times.

It reported that in private conversations at last year’s Tory Party conference in Birmingham, Mr Johnson said: “If you wanted to show your commitment to Europe, is it not time for us to have further and better economic integration with a road tunnel?

“That’s what we need.”

(Image: REUTERS)

Mr Johnson claimed such a plan had been ruled out in the 1980s, but he added: “That’s all changed.

“They now have the technology. You could come out of the EU but join Europe in the most fundamental way … You undo the damage done at the end of the Ice Age.”

The book, Fall Out: A Year of Political Mayhem, by the paper’s political editor Tim Shipman, revealed how Mr Johnson insisted: “The Channel is really a river whose tributaries used to be the Seine and the Thames.

“It became bigger and bigger as the ice melted until it separated Britain from France.”

(Image: REUTERS)

Reversing it would be “a great symbol of European commitment”, he said.

Building a road link between the Strait of Dover, the world’s busiest shipping lane, would hammer business on ferry lines and the Channel Tunnel, which took six years to build and opened in 1994.

The £9billion, 31.35 mile rail tunnel links Folkestone with Coquelles near Calais.

A total of 23.5 miles runs underwater - the longest undersea portion in the world.

At its lowest point, it is 250ft deep below the sea bed, and 380ft below sea level.

The tunnel was one of the biggest engineering projects ever undertaken in the UK.

More than 13,000 workers from England and France worked on the vast scheme.

Eleven boring machines, each weighing 1,110 tonnes, were used in the excavation works, according to Eurotunnel.

Its website says that in 1802, French mining engineer Albert Mathieu put forward a plan to “tunnel under the Channel, with illumination from oil lamps, horse-drawn coaches and an artificial island mid-Channel for changing horses”.