The UK shipping industry also poured scorn on the notion, saying: “Building a huge concrete structure in the middle of the world’s busiest shipping lane might come with some challenges.”
And the Foreign Secretary was accused of a “masterstroke in diversion” at the moment that the Brexit vote he inspired was “pulling up the drawbridge” from the major economies on the continent.
The backlash came after Mr Johnson floated the prospect of a crossing at Thursday’s UK-France summit, saying it was “ridiculous” that the two countries were linked by the Channel Tunnel only.
His aides claimed that, when he proposed building a second crossing of some form, Emmanuel Macron, the French President, replied: “I agree. Let’s do it.”
Mr Johnson had championed the idea of a road tunnel under the Channel, alongside the existing tunnel that carries rail passengers and freight, but has now put forward the bridge idea as well.
Asked for its view, Downing Street reacted coolly. The No 10 spokesman said: “I haven’t seen any plans on that.”
He added: “What was agreed yesterday, and what the Foreign Secretary tweeted about as well, is a panel of experts who will look at major projects together including infrastructure and we want to work very closely with our French colleagues on building a shared prosperous future.”
However, in his tweet, Mr Johnson had referred specifically to the Channel crossing, saying: “Our economic success depends on good infrastructure and good connections. Should the Channel Tunnel be just a first step?”
The Foreign Secretary is thought to be concerned that the tunnel will no longer be able to cope with increased demand, sometime in the next decade.
His aides have pointed out that bridges do exist across the world that are longer than the 26-mile distance between Dover and Calais.
However, Mr Johnson is renowned for putting forward grandiose infrastructure schemes which are quickly rejected, including a new airport in the Thames Estuary dubbed 'Boris Island'.
The French finance minister, Bruno Le Maire, said: “All ideas merit consideration, even the most far-fetched ones,” noting that the Channel Tunnel already linked Europe’s second and third-largest economies.
“We have major European infrastructure projects that are complicated to finance,” he told Europe 1 radio. “Let’s finish things that already under way before thinking of new ones.”
Nick Clegg, the former deputy prime-minister and Liberal Democrat leader, said: “It was a masterstroke in diversion from the fact that it’s Boris Johnson who is the prime architect of pulling up the drawbridge in terms of our economic relationship with France, and the rest of the continent, upon which the jobs and livelihoods of millions of people in this country depend.”
A joint declaration after the summit at the Royal Military Academy at Sandhurst, said: “The United Kingdom and France have a long history of collaboration in delivering cutting-edge technologies.
“Whether pioneering supersonic travel or better connecting our countries through the Channel Tunnel, cooperation between our nations has produced radical innovation.”
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