Boris Johnson's proposal for a bridge linking Britain to France has been greeted with scepticism by representatives of the UK's shipping industry.
The Foreign Secretary floated the idea of a cross-Channel bridge between during talks between the UK and France on Thursday.
He said "good connections" were important between the two countries, and suggested the Channel Tunnel might simply be "a first step."
Downing Street confirmed that the countries had agreed to establish a panel of experts to look at major projects - but added there were no "specific plans" for a bridge across the Channel.
French President Emmanuel Macron is reportedly enthusiastic about the idea.
Experts say that the project is "entirely feasible", potentially creating a stretch of tunnel in the middle to avoid impacting on one of the busiest shipping lanes in the world.
But the UK Chamber of Shipping said in a tweet: "Building a huge concrete structure in the middle of the world's busiest shipping lane might come with some challenges."
Among his comments, Mr Johnson also said: "It's crazy that two of the biggest economies in the world are connected by one railway line when they are only 20 miles apart."
Responding to the suggestion, a Downing Street spokesperson said: "I haven't seen any plans on that."
He added: "What was agreed yesterday, and I think that's what the Foreign Secretary tweeted about as well, is a panel of experts who will look at major projects together including infrastructure."
Shadow Foreign Secretary Emily Thornberry mocked the idea in a tweet, writing: "Who are these clowns claiming to run our country?"
However, bridge designer Ian Firth however, a past president of the Institution of Structural Engineers, said a bridge over the Channel was not as far-fetched as it may seem.
"It is entirely feasible. Before the Tunnel was built there were bridge options being looked at," he told BBC Radio 4's Today programme.
"There are bridges of a similar - if not quite the same - scale elsewhere. Of course this would not be one big span - the economics may lean towards something like 800m-1km spans.
"It would be a huge undertaking, but it would be absolutely possible, and shipping impact issues could be dealt with."
New Civil Engineer magazine technical editor, Dave Parker, told the programme that problems with shipping could be avoided by building artificial islands in the Channel, linked to the shore by viaduct and joined to one another by a tunnel.
The islands could become venues for attractions like hotels, casinos and duty-free shops, he suggested.
A joint declaration released after the summit at the Royal Military Academy, 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.
"We will continue to partner to invest in the technologies of tomorrow, including through a joint scheme, supported by up to 100,000 euro (£88,000) per year, to support academic exchanges, scientific collaboration, and innovation. "
The communique added: "As we look to the future of our relationship through the 21st century, we have agreed to establish a joint group of eminent and qualified persons to examine other options for future cooperation, including for significant projects."
Mr Johnson's proposal is the latest of grandiose projects which he has championed.
His call for a "Boris Island" airport in the Thames Estuary was rejected and plans for a garden bridge in central London were dumped by his successor as mayor, Sadiq Khan, on value-for-money grounds.
The Emirates cable car linking north and south London near the former Millennium Dome did get built, but has faced criticism over limited passenger numbers.