“Technically feasible – yes, but challenging, it is at the edge of what civil engineering has accomplished so far,” said Dr John McKinley, a senior lecturer in engineering at Queen’s University, Belfast, Northern Ireland.

If it is ever built, the bridge would cross rough water that is more than 300 metres deep in some places. It would need a series of supports to hold the structure in place, like the 7.8km Oresund bridge, which connects Sweden to Denmark.

Nor would it be a structure spanning the greatest expanse of open water. That prize goes the 36km Hangzhou Bay Bridge in China.

However, as well as the climatic challenges of the Irish Sea, and the depth, whoever builds it would face another obstacle that could literally blow Johnson’s scheme out of the water.

The bridge would have to cross – or if a bridge and tunnel system was adopted – go under Beaufort’s Dyke, Britain’s largest offshore dumping ground for conventional and chemical munitions disposed of after World War II.

The dyke lies just 11km off Portpatrick and according to Scotland’s Herald newspaper, the Ministry of Defence estimates there are 14,500 tonnes of artillery rockets filled with phosgene gas and two tonnes of concrete-encased metal drums, filled with radioactive waste, dumped there in the 1950s.

“The munitions dump is a huge unknown, and that uncertainty is the challenge. We just don’t know what is down there, where it is, and what condition it is in. Construction anywhere in the dyke would be risky,” McKinley told South China Morning Post.

The depth of the water would also make the building of the bridge extremely dangerous, some say impossible.

“The Hong Kong crossing doesn’t go across deep water and that is the big challenge if you are building deep foundations,” said Ian Firth, one of the world’s leading bridge engineers, responsible for the design concept of Stonecutters Bridge in Hong Kong.

“I won’t say it’s not feasible. What needs to happen is a proper study on how it could be done, whether it would be a floating structure or tunnelling. But the big issue is not a technical one, but economic, political and social,” he said.

Johnson believes the bridge could be a way of uniting the United Kingdom after Brexit. Scottish Nationalist Party leader Nicola sturgeon has poured cold water on the idea.

“I think to say there are some big questions around the feasibility and deliverability of the suggested bridge between Scotland and Northern Ireland would be an understatement,” she said. “If he wants to prove that this can be done and its feasible and deliverable then lets see how far he can get with it.”

IndyRef2: Brexit boosts Scottish movement to end London’s rule

When the idea was first mooted last year, Irish Prime Minister Leo Varadkar said he was not opposed as long as London pays.

But with a renewed impetus for Scottish independence following the UK’s departure from the EU and the growing possibility of Irish reunification following the strong showing of the one nation Sinn Fein party in elections north and south of the island recently, there is the distinct possibility the union crumble before the bridge is even built.

And what about the cost? The current estimate is £20 billion (US$26 billion), a figure that could soar once work begins. Supporters point to the privately financed Channel Tunnel that links the UK to France, but a fast train from London to Paris and Brussels and the whole of the European continent has far more commercial potential than a road bridge linking Glasgow to Ulster.

And on a windy day, would people want to drive across?

Boris Johnson resorts to publicity stunt as UK election becomes ‘tight fight’

“Bad weather is an issue of you are driving but these days we can use wind shield – whether it would be pleasurable that is another question,” said Firth.

“The economics doesn’t make sense,” said McKinley.

“At £20 billion that would be about two-thirds of Northern Ireland’s annual GDP, or about £13,000 per head of population. Even that estimate is approximate. It might cost £15 billion, it might cost £20 billion, it might cost £60 billion. I wouldn’t want to guess.”

The bridge is by far one of the prime minister’s boldest ideas to invest some vigour into the UK economy.

Johnson owes his 80-seat majority election victory last December to former Labour voters in the post-industrial heartlands of the Midlands and the North. He has promised to invest billions of pounds in developing much needed transport infrastructure in those regions.

With even the conservative party deeply divided on the issue, the project, costs of the £36 billion when it was first mooted have already soared to £100 billion and the final bill could be as much as £200 billion.

“As everybody knows, the cost forecasts have exploded. But poor management to date has not detracted in my view from the fundamental value of the project,” Johnson told Parliament on Tuesday when he announced his decision to continue with the project.

Johnson said he would be appointing a minister whose sole job was to look over the progress of HS2, which would be completed by the end of the decade.

He also announced a multibillion pound package to deliver at least 4,000 new “British made” zero emission buses and over 400km of new cycle routes.

In his former position as mayor of London, Johnson famously spent £45 billion commissioning a pedestrianised garden bridge across the River Thames that never materialised.

Johnson’s proposed 35km bridge between UK and France met with ridicule

This has led to criticism that he was driven by “vanity projects” – but supporters of the prime minister say vision and ambition are exactly what is required in the UK as it breaks away from the EU.

“Two centuries ago our ancestors could have been content with breeding faster horses.

Instead, they invented the railways.” Johnson told Parliament.

“They created the transport network on which the United Kingdom rose to economic pre-eminence. They looked to the future of transport and they made it happen and today it is our duty to do the same.”

The number-crunching will be left to the Chancellor of the Exchequer Sajid Javid, who will present his post-Brexit budget next month along with the details of the National Infrastructure Plan.