The British Prime Minister Boris Johnson has put a price on his plan for a bridge between Northern Ireland and Scotland.

"It would only cost about £15bn [€16.7bn]" he told a group of children as he visited a ship on London's River Thames on Thursday.

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."

He has reportedly asked British government officials for advice on the costs and risks of such a project.

He first suggested the idea while serving as UK foreign secretary, telling the UK Sunday Times last year: "What we need to do is build a bridge between our islands. Why don't we? Why don't we?

British Prime Minister Boris Johnson visits the NLV Pharos, a lighthouse tender moored on the River Thames, to mark London International Shipping Week | Image: Daniel Leal-Olivas/PA Wire/PA Images

"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 at Westminster, 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.

However Sinn Féin leader Mary Lou McDonald has said the idea of a bridge between Northern Ireland and Scotland is just a "distraction" from Brexit talks.

"I don't think the 'Boris bridge' is a real proposal.

"There are so many huge infrastructural needs on the island, I don't think we should get lost in a distraction about a bridge."

"Although one would have to say that there is something attractive in the notion of a bridge between an independent Scotland and a united Ireland."