Boris Johnson has hinted that he wants to build a 25-mile bridge across the Irish sea between Northern Ireland and Scotland.
The Prime Minister has repeatedly backed the idea of a bridge across the Irish Sea - despite his plan for London's much smaller Garden Bridge ending in expensive failure.
And experts have raised concerns about the cost and feasibility of it.
Today DUP MP Ian Paisley asked the Prime Minister to build a "Boris bridge" between Northern Ireland and Great Britain.
Documents leaked in September revealed that officials had the Treasury and the Department for Transport were asked to look at risks and costs of the £15billion project.
But experts have warned the sea channel between Portpatrick and Larne is almost 1,000ft deep and may contain unexploded WW2 bombs.
James Duncan, retired offshore engineer from Edinburgh, told The Sunday Times that this idea was "about as feasible as building a bridge to the moon"
But responding to the DUP MP today the Prime Minister indicated his support for the project.
“As for his desire for a bridge to connect the two biggest isles in the British Isles, all I can say is it is a very interesting idea. I advise him to watch this space and indeed watch that space between those islands. What he has said has not fallen on deaf ears.”
Boris Johnson's previous attempt at building a bridge was beset by controversy from the start.
The Garden Bridge was finally scrapped by his successor Sadiq Khan in 2017 after costs spiralled.
Critics blasted the link for being privately run, yet publicly-subsidised, while there was a more pressing need for Thames crossings elsewhere.
Yet Boris Johnson was a doughty defender of the “vanity project” - even making a secretive trip to San Francisco in 2013 in a bid to get Apple to sponsor it.
Architect Prof Alan Dunlop has said a bridge between Scotland and Northern Ireland could fuel a "Celtic powerhouse" and cost around £15bn.
Prof Dunlop said while a link from the Mull of Kintyre would be shorter and cheaper, a bridge of about 25 miles from Portpatrick in Scotland would provide a bigger boost.
It comes as the PM's top Brexit negotiator David Frost holds further talks with EU officials in Brussels, aimed at breaking the deadlock.
Mr Johnson is said to be preparing to soften his position on the Irish Backstop - either to seek a repeatedly rejected "time limit", despite ruling it out, or an "all-Ireland" deal that could create checks between Northern Ireland and Britain.