A bridge between Northern Ireland and Scotland would be “very good” and would “only cost about £15 billion”, according to the Prime Minister.

So how likely is it that people will be driving across the Irish Sea in the near future? Here are the answers to some questions surrounding the idea.

– Where would the bridge go?

There are a couple of potential routes when you look at a map. A bridge could possibly run from the historic port of Larne in Co Antrim to Portpatrick in Dumfries and Galloway in Scotland.

Another option is from rugged Torr Head, further north on the Antrim coast, to the Mull of Kintyre, while Boris Johnson himself has suggested a route between Larne and Stranraer, with Stranraer being about a 15-minute drive across land from Portpatrick.

– How much would it cost?

The cost would of course depend on how long the bridge is. The distance from Larne to Portpatrick is around 45km, while the distance from Torr Head to the Mull of Kintyre is around 20km. Mr Johnson said it would “only cost about £15 billion”.

But others have reportedly suggested that £20 billion would be a conservative estimate for any future bridge linking the two countries. So, as for how much it would cost, the only certainty at this stage seems to be that a project of this scale would run into many billions of pounds.

– What problems could the ambitious proposal face?

There is no problem with distance, money or Beaufort’s Dyke explosives disposal area, according to the Prime Minister. In November 2018, he said: “It is a far shorter distance than that covered by some bridges these days – look at Shanghai to Ningbo.

The problem is not the undersea Beaufort’s Dyke or lack of funds. The problem is an absence of political will.” But concerns may be raised about the route due to the wartime dumping of vast amounts of munitions, particularly in Beaufort’s Dyke, a deep seabed trench between Scotland and Northern Ireland.

Beaufort’s Dyke was also revealed to be the location for the dumping of radioactive waste during the 1950s and early 1960s.

– How serious is the Prime Minister about the bridge idea?

Mr Johnson has reportedly asked Government officials for advice on the costs and risks of such a project. He first mooted the idea while serving as foreign secretary, telling The Sunday Times last year: “What we need to do is build a bridge between our islands. Why don’t we? Why don’t we?”

The DUP – which props up the Prime Minister’s minority government – supports proposals for a bridge which it hopes could bring economic benefits to Northern Ireland and Scotland, but it has also been touted as a potential solution to the controversial backstop.

– Does Boris Johnson like bridges?

It would appear he does. The possible bridge between Northern Ireland and Scotland is not the first bridge Mr Johnson has suggested or been associated with. In January 2018, he raised the prospect of a bridge spanning the English Channel following a top-level summit attended by the then prime minister Theresa May and French President Emmanuel Macron.

His idea split opinion but experts said it was technically possible. As London mayor, he supported the Garden Bridge project which was proposed by actress Joanna Lumley and abandoned amid bitter recriminations over the “waste” of taxpayers’ cash.

The plans were effectively killed off by Mr Johnson’s Labour successor, Sadiq Khan, when he refused to provide guarantees for the costly crossing in the centre of London on value-for-money grounds. A City Hall source claimed the bridge was a “Boris vanity project”.

– Has the Prime Minister championed any other projects that have either failed to get off the ground or faced criticism?

Yes. Mr Johnson’s 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, while 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.