Boris Johnson is plunged into a new controversy after the idea of a bridge between Scotland to Northern Ireland was "very good" and would cost £15bn to build.

The Prime Minister told a group of schoolchildren that he had discussed a crossing over the North Channel this week, which has been suggested as a possible solution to ensuring an open border with the Republic of Ireland after Brexit.

If built, it could be around 28 miles long, crossing the North Channel near the town of Portpatrick in Galloway to either Larne or Donaghadee in Northern Ireland.

But the leader of the SDLP, which advocates Irish unification called the bridge "a distraction tactic" and Sinn Fein said it was "fantasy politics".

SDLP infrastructure spokeswoman Sinead Bradley accused Mr Johnson of having an "obsession with undeliverable projects" and slammed the "fantasy bridge" as a mere distraction to the chaos at Westminster.

READ MORE: Boris Johnson seeks cost estimates for a Scotland-Northern Ireland bridge

"Conservative and Democratic Union Party MPs are tripping over themselves to talk about the fantasy bridge to Scotland," she said.

"It's hard to blame them for wanting to talk about anything other than their government which appears to be on fire but this really is a pathetic attempt at distraction."

Speaking on Thursday, Mr Johnson said: "(I was talking yesterday) about building a bridge from Stranraer in Scotland to Larne in Northern Ireland - that would be very good."

The DUP reportedly believe that committing to build a bridge physically connecting the mainland with Northern Ireland might help solve the current impasse in Westminster by reducing the threat of a post-Brexit border across the Irish sea under any new deal — a fact that Johnson himself foresaw last year.

HeraldScotland:

Economist Dr Esmond Birnie of Ulster University has said the economic boost the bridge might add would come nowhere near to the cost of its construction and maintenance.

And renowned engineer James Duncan said the chances of pulling the project off without a problem would be "about as fanciful as building a bridge to the Moon".

Channel 4 reported on Tuesday that it had obtained documents revealing that the Prime Minister has requested guidance from the Treasury and Department for Transport on the possible costs and risks of such a project.

READ MORE: Boris Johnson seeks cost estimates for a Scotland-Northern Ireland bridge

The PM reportedly asked civil servants to find out “where this money could come from” and “the risks around the project”.

Mr Johnson first floated the bridge idea last year, when he was Foreign Secretary.

The Department for Transport produced a “factual paper on the subject” after conversations between the DUP and former Secretary of State for Transport Chris Grayling.

Last year,  leading Scottish architect, Alan Dunlop  proposed building a bridge between Scotland and Ireland to create a ‘Celtic Powerhouse’

The professor of architecture at Aberdeen’s Robert Gordon University said that such a crossing could be created for about £12 billion.

Mr Dunlop said: ‘To propose a bridge between Scotland and Ireland would be a big step in creating a Celtic Powerhouse and give politicians the opportunity to invest in the infrastructure of the true North.’

He said a crossing from Mull of Kintyre to Torr Head would cost about £12 billion, but that a bridge between Portpatrick and Bangor or Larne would have more benefits, despite costing more.