Boris Johnson is stepping up work on his plan to build a £20bn bridge between Scotland and Northern Ireland, despite experts dismissing it as “clueless” and “bonkers”.

A “proper piece of work” is under way in Whitehall, his spokesman said, arguing it proved the prime minister is “ambitious in terms of infrastructure projects”.

“This is an idea that the prime minister has expressed interest in in the past and I think he said at the time ‘watch this space’,” the spokesman said.

“Government officials are carrying out work in relation to the idea of a bridge linking GB mainland to NI. There’s a proper piece of work being carried out into the idea.”

The proposal – first mooted by Mr Johnson during last year’s Tory leadership campaign – enjoys strong Unionist support in Northern Ireland, but provoked deep scepticism in Scotland.

But experts have said that, while technically possible, spanning the 26-mile stormy stretch of water between Portpatrick and Larne would be fraught with problems, which means the costs could spiral.

It would require dozens of support towers – at unprecedented depths of more than 1,000ft in places – while navigating Beaufort’s Dyke, a submarine trench containing more than 1 million tonnes of Second World War munitions.

One engineer called it “bonkers” to try to put a price on such a project, while Chris Wise, the engineering designer of the 2012 Olympic velodrome, said last year: “It’s socially admirable but technically clueless.”

Andrew Adonis, the former Labour transport secretary, said of No 10’s latest defence of the scheme: “Yet the government is about to delay the HS2 line to Manchester and Leeds. Get real.”

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Notably, the spokesman declined to say who exactly was carrying out the scoping work, how many people were working on the project, or when it might be completed.

He also declined to respond to the suggestion it could end up being a bridge between two countries outside the UK – if Scotland votes for independence and Brexit drives Northern Ireland into unity with the Republic.

The criticism of the idea has also noted Mr Johnson’s past failures with bridge proposals, including a similarly ridiculed call for a crossing between England and France.

Most notoriously, more than £40m was spent on the planned Garden Bridge in London, before it was finally abandoned by Sadiq Khan, his successor as the capital’s mayor.

Nicola Sturgeon, the Scottish first minister, said that, if the government had a spare £20bn, Edinburgh and Belfast could suggest better ways of spending the money.