Boris Johnson has said that a land bridge between Northern Ireland and Scotland was a "very good" idea that would cost £15bn to build.
The Prime Minister told a group of schoolchildren that he had discussed a crossing over the Irish Sea this week, which has been suggested as a possible solution to ensuring an open border with the Republic of Ireland after Brexit.
It emerged officials are actively studying such a plan after Channel 4 News obtained documents showing both the Treasury and Department for Transport had been asked to look into the costs and risks of the project.
The PM reportedly wants to find out where the money for the project, backed by the DUP, could come from, as well as "the risks around the project" - which apparently include "WW2 munitions in the Irish Sea".
Speaking today aboard a lighthouse tending ship moored on the Thames, Mr Johnson said: "(I was talking yesterday) about building a bridge from Stranraer in Scotland to Larne in Northern Ireland - that would be very good.
"It would only cost about £15 billion."
Mr Johnson has previously spoken of his support for the idea, 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?
"There is so much more we can do, and what grieves me about the current approach to Brexit is that we are just in danger of not believing in ourselves, not believing in Britain."
And the PM was said to have floated the idea of a 22-mile bridge across the Channel to France to enhance transport links with Europe after Brexit.
He is said to have discussed the idea with French president Emmanuel Macron when he was foreign secretary at the Anglo-French summit in January 2018.
In response to the reports a bridge could be built earlier this week a Government spokesperson said: "Government regularly commissions work to examine the feasibility of projects.
"During the leadership campaign candidates spoke about a number of issues which resulted in Number 10 commissions ahead of a new Prime Minister taking over.
"This PM has made no secret of his support for infrastructure projects that increase connectivity for people and particularly those that strengthen the Union."