Boris Johnson has said a bridge between Northern Ireland and Scotland would be "very good" and would cost around £15 billion.
The Prime Minister revealed his thoughts on the project to London schoolchildren, during a visit to the river Thames to mark International Shipping Week.
Speaking aboard the NLV Pharos, a Coast Guard vessel moored on the river, he told the children about the possibility of building a bridge over the Irish Sea.
He 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."
It follows reports that Mr Johnson has asked Government officials for advice on the costs and risks of such a project.
The Democratic Unionist Party (DUP) which props up the Tory Government with its confidence and supply arrangement, allegedly supports the proposal.
They hope such a bridge would bring economic benefits to Northern Ireland and Scotland, according to the Press Association.
It has also been touted as a potential solution to the Brexit impasse by serving as an alternative to the controversial Irish backstop.
The now-Prime Minister first floated the idea while serving as foreign secretary under his predecessor Theresa May.
He told 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."
During his stint in the Foreign Office, Mr Johnson also reportedly suggested building a bridge across the English Channel to connect the UK and France
