Irish Sea bridge: Scoping work begins on Boris Johnson's 'ambitious' idea
Government officials are examining the potential for a bridge linking Scotland and Northern Ireland, after the prime minister touted the idea late last year.
Despite critics saying it would be a waste of time, Boris Johnson's official spokesman said the project was a "proper piece of work" that a "range of officials" were already looking into.
Downing Street said Boris Johnson - who is known to be a fan of bridges - was "ambitious" about infrastructure projects and is "looking at a wide range of schemes across the UK which could improve connectivity."
Mr Johnson has repeatedly spoken about the prospect of a bridge, even though experts have warned that the depth of the Irish Sea and the presence of dumped munitions would cause problems for any project.
The project could cost a reported £20 billion, although Mr Johnson has previously said it would "only cost about £15 billion".
The Prime Minister's official spokesman said: "The PM 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."
The scoping work is being run from Number 10, with a range of officials reporting into it.
Irish Sea Bridge: Boris Johnson puts price on linking Scotland to Northern Ireland
More than £40m of taxpayers money 'wasted on London's failed Garden Bridge'
Boris Johnson's proposal of bridge across the Channel met with mixed response
The Prime Minister told MPs "watch this space" when asked about the prospect of a "Boris bridge" in Parliament in December 2019.
The distance from Larne to Portpatrick, one of the most likely routes for a bridge, is around 45km.
There is no problem with distance, money or the Beaufort's Dyke explosives disposal area, according to the Prime Minister.
In November 2018, he said: "The problem is not the undersea Beaufort's Dyke or lack of funds. The problem is an absence of political will."
The new plan is the latest in a list of bridge projects suggested by the prime minister.
When London Mayor he infamously began a huge project which would have seen a "Garden Bridge" - decorated with dense flora - erected across the River Thames - the plan was eventually abandoned.
Then, when Theresa May's foreign secretary he suggested building a bridge across the English Channel which would connect Britain with France - the project never got off the ground.
On the latest bridge, government sources say it will be possible to build it with a tunnelled section to cope with some of the difficulties caused by the depth of the Irish Sea.