A £20 billion road tunnel between Scotland and Northern Ireland rather than a bridge spanning the Irish Sea is being promoted by senior figures within the UK Government to increase connectivity across the UK and bolster the Union.

But Government critics dismissed the idea as "outrageous" and a "vanity project".

The confirmation by Downing St that officials are undertaking a “proper piece of work” on the possibility of a new Irish Sea link comes as Boris Johnson today prepares to give the green light to the controversial £106bn HS2 rail project as part of an “infrastructure revolution”.

This will also include a £5bn plan for better bus and cycle links south of the border.

The idea of a “Boris Bridge,” has, according to Whitehall sources, already received a positive response both from Northern Ireland’s Democratic Unionists and Leo Varadkar, the Irish Prime Minister.

One suggested model for it is the Oresund Bridge and Drogden Tunnel, connecting Denmark and Sweden.

However, one senior Government insider told The Herald that a road tunnel would be more appropriate to link, say, the 20 miles across the Northern Channel between Portpatrick on the west coast and Larne in County Antrim than a bridge open to the elements. “Think tunnel,” he declared.

Yet a road tunnel would present its own challenges. In 1999, 39 people died after a fire broke out in the trans-alpine road tunnel at Mont Blanc. This is seven miles long; one under the Northern Channel between Scotland and Northern Ireland would be three times longer.

The biggest concern in establishing any Irish Sea link would be over Beaufort's Dyke, which contains a million tons of munitions left over from World War Two together as well as two tons of radioactive waste dumped there in the 1950s.

But whether a Boris Tunnel or a Boris Bridge, the SNP branded the exercise a “vanity project”.

Nicola Sturgeon said her mind was not closed to the idea of a new Irish Sea link but the First Minister stressed: “If he has got £20bn to build such a bridge going spare at the moment, that could be spent on more important priorities."

Liberal Democrat MP Christine Jardine said: "It's an outrageous proposal. I remain to be convinced it's even possible."

Labour peer, Andrew Adonis, the Government’s former infrastructure tzar, tweeted a reference to Mr Johnson’s failed Garden Bridge project in the UK capital when he was London Mayor: “So now Boris wants to build a garden bridge all the way to Belfast. If you think HS2 has problems, multiply by 10 and divide the benefits by 100.”

The shipping industry also had qualms, suggesting a new Irish Sea link could threaten the livelihoods of ferry operators.

A UK Chamber of Shipping spokesman said the industry supported any initiative to improve trade and tourism links.

“However, there are already a range of ferry operators taking tourists and trade between Northern Ireland and Scotland. Spending £15bn to £20bn of taxpayers’ money on a bridge simply to replicate what those ferries already do is unnecessary. The money could be far better spent improving road and rail links to our ports across the UK.”

Asked about the prospect of a Boris Bridge, the Prime Minister’s spokesman pointed out how Mr Johnson had suggested the idea “could have some merit,” so people should expect the Government to be looking into it.

Downing St officials have begun working with others across Whitehall in a “scoping” operation, the details of which would be presented to Mr Johnson to see if the scheme should proceed further.

In 2018 when the PM first broached the idea of a bridge across the Irish Sea, he said: “The problem is not the undersea Beaufort's Dyke or lack of funds. The problem is an absence of political will."

The spokesman said Mr Johnson was “ambitious” about infrastructure projects and that a wide range of work was going on to improve connectivity across the UK and, what some have also dubbed the Union Bridge, was one example of it. “A proper piece of work is being carried out into this idea,” he explained.

Bolstering the Union with infrastructure projects will also be a key theme of the HS2 announcement. While the go-ahead with be confirmed for the first phase London to Birmingham link, so too will be the second phase continuing it to Crewe and connecting it to West Coast Main Line, which will benefit passengers travelling to Glasgow and Edinburgh.

Alongside this, Mr Johnson is expected to confirm funding for so-called Northern Powerhouse Rail, which will incorporate a “Crossrail of the North,” as part of his grand plan to “level up” connectivity across the whole of the UK.

In a Commons statement, the PM will announce bus services across England will be transformed with simpler fares, thousands of new buses, improved routes and higher frequencies.

And he will tell MPs cycle routes will see a major boost with the construction of more than 250 miles of new, high-quality separated cycle routes south of the border.

“Local transport connections have a truly transformative role to play in levelling up infrastructure across the country,” declared Mr Johnson.

Daily journeys for work or leisure were about so much more than just getting from A to B; they were the key to accessing skilled jobs, boosting businesses and unlocking economic growth.

“That’s why improving connectivity by overhauling bus services and making cycling easier than ever is such an important step forward, to make sure every community has the foundations it needs to thrive,” he added.

But Andy McDonald for Labour said: “HS2 has been appallingly mismanaged by the Conservative Party, which has failed to deliver a single major infrastructure project on time or within budget.

“If HS2 is going to get back on track, the project has to be integrated with Crossrail for the North, it needs to be managed as part of an advanced rail network, and it must eventually extend into Scotland so that we remove the need for domestic flights.”

The Shadow Transport Secretary stressed HS2 should be run under public ownership. “That way it is the UK public – not private or overseas state-owned companies – who will see a return on this investment.”