Boris Johnson has said building a bridge between Scotland and Northern Ireland is a “very good” proposal that would “only cost about £15bn”.
The Prime Minister revealed his thoughts on the idea to a group of schoolchildren playing with a model container ship at an event in London on Thursday.
It comes after media reports Johnson had been considering building a bridge across the Irish Sea – with a suggestion it could even fix the Brexit deadlock over the Irish backstop.
The PM told the children: “(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.”
Johnson is reported to have asked Treasury and transport department officials for advice on the costs and risks of such a project.
There has been no contact between the Scottish and UK governments on the idea, with the former pointing to “an obvious number of practical obstacles and challenges” to delivering it.
A Scottish Government spokesman said: “We are always keen to talk about how we can strengthen connections between Scotland, Northern Ireland and the Republic of Ireland.
“There are an obvious number of practical obstacles and challenges to such a concept, and it would require a robust assessment of the costs or benefits of such a project in the first instance.”
When he was London mayor, Johnson backed an ill-fated proposal to build a “garden bridge” across the Thames, eventually ditched by his successor Sadiq Khan.
The botched project, which got to the stage of handing out construction contracts, wound up costing the taxpayer £43m.