Boris Johnson has put a price on his "very good" idea for a bridge between Northern Ireland and Scotland.
"It would only cost about £15bn," the prime minister told a group of children as he visited a ship on the Thames.
Mr Johnson told the pupils, who were playing with a model container on board the lighthouse tender NLV Pharos, that he had recently been talking about 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 £15bn."
The PM has reportedly asked government officials for advice on the costs and risks of such a project.
He first suggested 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 has also reportedly suggested building a bridge across the English Channel to link France with the UK.
Northern Ireland's DUP, which supports Mr Johnson's minority government in key parliamentary votes, support proposals for a bridge with Scotland.
The party hopes it would bring economic benefits to both countries.
The bridge has also been touted as a potential solution to the Brexit backstop issue.
Part of the withdrawal agreement Theresa May negotiated with the EU, the backstop is an insurance policy designed to avoid the return of a hard border on the island of Ireland.
But the DUP and many Conservative MPs are opposed to it, because it would mean adhering to EU rules and regulations for an indefinite period.