A “proper piece of work” is being carried out by the UK Government into the feasibility of building a bridge between Scotland and Northern Ireland to help UK connectivity and underpin the importance of the Union, No 10 has confirmed.

Already dubbed “the Boris Bridge,” the project could span the 20 miles between Portpatrick in Scotland and Larne in County Antrim and has been estimated to cost around £20 billion.

But the SNP was swift to brand the idea a "vanity project".

Asked about the prospect of such a bridge, the Prime Minister’s spokesman said: “It’s an idea which the PM has expressed an interest in…He set out this was an idea which he believed could have some merit so, as a result of that, you would expect the Government to be looking into it."

It is understood that officials in Downing St have begun working with others across Whitehall in a “scoping” operation to see if such a project would be viable and, after No 10 pulled the preliminary work together, it would then be presented to Mr Johnson to see if the scheme should proceed further.

READ MORE: Prime Minister brands idea of 'Boris bridge' between Scotland and Northern Ireland 'very interesting'

No details were given as to who was in charge of the work or when it would report back to the PM but it is expected a report would be on his desk in No 10 by the end of the year.

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.

Whitehall sources stressed that not only had Northern Ireland’s Democratic Unionists expressed support for such a plan but so too had Leo Varadkar, the Taoiseach. “It would be wrong to portray this as a Nationalist idea,” stressed one Government insider.

However, it has already been pointed out that the difficulties of building a bridge across the North Channel of the Irish Sea could be too great.

Apart from the weather, one major problem would be Beaufort's Dyke, Britain’s largest offshore dumping ground for conventional and chemical munitions post World War Two.

The dyke lies just seven miles off Portpatrick and the Ministry of Defence estimates there are a million tons of munitions at the bottom of this deep trench, including 14,500 tons of artillery rockets filled with phosgene gas as well as two tons of concrete-encased metal drums filled with radioactive waste dumped there during the 1950s.

However, engineers are believed to have come up with the idea of spanning the channel using a bridge and a tunnel to get round the problem of the dyke.

One idea is for the Boris Bridge to run from the Scottish coastline over Beaufort’s Dyke before becoming a tunnel for the final stretch to Northern Ireland; two artificial islands might be needed to help the bridge span the channel.

This plan would be modelled on the Oresund Bridge, which runs for five miles from the Swedish coast near Malmo to an artificial island in the middle of the Oresund Strait. It then turns into a 2.5-mile tunnel to the Danish island of Amager, near Copenhagen.

The Oresund crossing was the setting for The Bridge, the popular BBC series starring Sofia Helin and Killing Eve actor Kim Bodnia as detectives investigating a gruesome murder.

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

The length of the bridge should not be a problem for engineers.

The Hong Kong-Zhuhai-Macau bridge-tunnel system crosses 30 miles of water while the structure, which spans the greatest expanse of open sea is the 22 mile Hangzhou Bay Bridge in China.

However, these are dwarfed by the world's longest bridge, again in China, the Danyang-Kunshan Grand Bridge on the Beijing-Shanghai railway, which is 102 miles long.

READ MORE: Boris Johnson seeks cost estimates for a Scotland-Northern Ireland bridge

Two years ago following a UK-France summit, Mr Johnson, the then Foreign Secretary, floated the idea of another bridge; one from the UK to France, noting how the Channel Tunnel should be regarded as merely "a first step" in the physical links uniting the two countries.

But Labour’s Emily Thornberry branded the PM a “clown” for suggesting it.

And Labour peer Lord Adonis, the former UK Government infrastructure tzar was also dismissive, saying: “Boris is proposing this latest fantasy just as he sets about destroying our most important bridge to Europe: the European Union.”

Some might argue that the PM does not have a good track record on building bridges. His notorious idea for a 366-metre Garden Bridge in London, branded an “embarrassing mess,” was never realised yet cost the public purse £43 million.

The SNP dismissed Mr Johnson's Union Bridge plan.

“It is going to take more than a bridge to undo the harm to EU relations that the Tory Government has caused with its extreme Brexit plans and given Boris Johnson’s failed history of unwanted and over budget bridge projects we are going to take a lot of convincing," declared a party spokesman.

“The SNP will always welcome engagement with how we can strengthen relations with Northern Ireland and Ireland but we will focus on practical and achievable ideas; not unsubstantiated vanity projects and baseless briefings which this Tory Prime Minister is all too familiar," he added.

Plans for a link between Scotland and Northern Ireland go back to 1869 when Luke Livingston Macassey, an Irish engineer, argued it would “tend to the consolidation of empire, so greatly desired by all lovers of order and prosperity”.

Yet the Scottish Government has previously expressed a deal of scepticism about the idea, citing an “obvious number of practical obstacles and challenges” and the need for a “robust assessment of the costs or benefits of such a project”.

It seems No 10, under orders from Mr Johnson, has now begun just that.