BORIS Johnson has been branded a “clown” over his proposal for a cross-Channel bridge as Theresa May distanced herself from the idea.

The Foreign Secretary raised the prospect of a second crossing to link the UK to mainland Europe following a top-level summit attended by the Prime Minister and French President Emmanuel Macron at Sandhurst on Thursday.

Mr Johnson said "good connections" were important to the relationship and raised the idea that the Channel Tunnel should be regarded as merely "a first step" in the physical links uniting them.

He is understood to want a new fixed link between the UK and France and believes "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".

Sources close to the Secretary of State claimed Mr Macron was enthusiastic about the prospect of a new UK-France link.

However, Emily Thornberry, the Shadow Foreign Secretary, did not share the enthusiasm, tweeting: "I ignored this earlier because I assumed it wasn't real. Apparently it is. I mean...who are these clowns claiming to run our country?"

And Downing Street, when asked about Mr Johnson’s idea, failed to warm to it.

Mrs May’s Deputy Spokesman made clear: “I have not seen any plans on that but what I would say is we are going to have very close ties with France economically, culturally and in areas like defence and security for many many decades to come.

“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.”

Asked if the panel of experts could look at the possibility of building what has already been dubbed the “Boris bridge,” the deputy spokesman said: “As I have said I have not seen any plans for that.”

Asked if Mr Johnson told the PM about his idea, he replied: “I have not seen any plans for that...”

Asked if Mrs May thought the idea was good in principle, the deputy spokesman said: “I have not seen any plans for a bridge.”

Asked if the Foreign Secretary’s idea had come as a surprise to the PM, he added: “Look, the Foreign Secretary set out in his tweet, something which was set out more formally on this panel of experts to look at major projects together and that’s something we support.”

The spokesman could not provide details of who would sit on the infrastructure panel or what it would look at. He added the PM thought Mr Johnson was doing an "excellent job".

Yet his suggestion that a new UK-France link could come in the form of a bridge over the Channel won a sceptical response from representatives of the shipping industry.

The trade body, 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."

The Road Haulage Association said the costs and practical implications of a bridge across the Channel would be "enormous".

Richard Burnett, its Chief Executive, said: "We're better off spending smaller amounts of money on improving our crumbling roads and opening more lorry parks.

"The Strait of Dover is the world's busiest shipping lane carrying more than 500 ships daily, so construction would cause huge disruption to sea traffic. And what of the impact on the road network and people of Kent?”

He added: "The Channel Tunnel and the ferry routes are working well within capacity, so it makes no sense to commit huge amounts of taxpayers' money in an uncertain economic climate to a costly bridge project that we don't need."

However, bridge designer Ian Firth, a past president of the Institution of Structural Engineers, said a cross-Channel bridge was not as far-fetched as it might seem.

He told BBC Radio 4's Today programme: "It is entirely feasible. Before the Tunnel was built there were bridge options being looked at.

"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.”

He added: "It would be a huge undertaking but it would be absolutely possible and shipping impact issues could be dealt with."

Dave Parker, the technical editor of New Civil Engineer magazine, 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 after the summit at the Royal Military Academy in Berkshire 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, co-operation 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 co-operation, including for significant projects."

Mr Johnson's bridge proposal is not the first grandiose project he has championed.

His call for a "Boris Island" airport in the Thames Estuary was rejected by an inquiry looking into the expansion of air capacity in the South East 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.

The Foreign Secretary’s Conservative colleague Charlie Elphicke, who represents Dover, said: "Boris is right. We absolutely must invest in infrastructure to keep trade flowing between Britain and France.

"Let's start by dualling the A2 to Dover, building the Lower Thames Crossing and lorry parks on the M20. Surely it's not a bridge too far for the Government to invest in Kent?"