BORIS Johnson has doubled down on the idea of building a bridge between Scotland and Northern Ireland, saying it would “only cost about £15 billion”.

The Prime Minister revealed the figure as he spoke to children in London about the “very good” plan.

He said he had been talking the day before “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."

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When he was mayor of London, Mr Johnson started work on a "garden bridge" across the Thames which came to nothing and cost taxpayers £43m.

It emerged earlier this week that the PM had asked government officials for advice on the costs and risks of a bridge across the 30-mile wide North Channel.

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The DUP support a bridge in order to boost Northern Ireland’s economy.

It has also been touted as a potential solution to the controversial backstop.

Mr Johnson first floated the idea while serving as Foreign Secretary last year.

He told the Sunday Times: "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.