Boris Johnson has said a bridge linking Northern Ireland and Scotland would be "very good", after reports revealed the government had considered an Irish Sea Bridge.
The prime minister revealed his thoughts to schoolchildren, telling them the ambitious engineering project would cost an estimated £15 billion.
He on onboard lighthouse tender NLV Pharos on the Thames when he told the children that he had recently been discussing the possibility of constructing the bridge.
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.”
Channel 4 reported the "the risks around the project” appear to include "WW2 munitions in the Irish Sea".
The DUP – who prop up the Prime Minister’s minority government – support proposals for a bridge which they hope could bring economic benefits to Northern Ireland and Scotland, but it has also been touted as a potential solution to the controversial backstop.
Mr Johnson first mooted the idea while serving as foreign secretary, 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.”
Mr Johnson last year reportedly suggested also building a bridge across the English Channel to connect the UK and France.
But that wasn't Mr Johnson's first ambitious bridge project - as London mayor in 2014 he approved a scheme to build the now infamous Garden Bridge.
The project incurred huge costs and was eventually scrapped by new mayor Sadiq Khan after the project had spent £53.5m without even beginning construction.
Mr Johnson also put his weight behind a proposal dubbed "Boris Island", which would have seen an airport build in the River Thames Estuary.
The project was rejected by the airport commission in January 2014.