UK Prime Minister Boris Johnson has reportedly asked officials to look at plans to build a bridge between Scotland and Northern Ireland.

One retired offshore engineer previously branded the plan — which Johnson first floated last year when he was foreign secretary — a "thoughtless soundbite" that "no sane contractor or responsible government" would sanction.

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UK Prime Minister Boris Johnson is said to be exploring plans to build a bridge between Scotland and Northern Ireland.

Johnson has asked government officials to look at the costs and risks of building a 28-mile bridge across the Irish Sea, Channel 4 News reported Tuesday.

The prime minister has reportedly asked officials to examine "where this money could come from" as well as "the risks around the project," one of which appears to be unexploded bombs dropped into the sea during World War II.

Johnson first floated the idea of a Brexit bridge across the Irish Sea when he was foreign secretary in 2018 and has apparently revived it since entering Downing Street.

His proposal was, at the time, branded by one expert as a "thoughtless soundbite" that "no sane contractor or responsible government" would sanction.

Writing to The Sunday Times, James Duncan, a retired offshore engineer from Edinburgh, said the idea was "about as feasible as building a bridge to the moon."

"Many long bridges have been built, but none across such a wide, deep and stormy stretch of water," he continued.

"For a great part of the 22-mile route the water is more than 1,000 feet deep. It would require about 30 support towers at least 1,400 feet high to carry the road deck across the deepest part and above the shipping channel. In total the bridge would require 54 towers, of heights never achieved anywhere in the world."

The Northern Irish Democratic Unionist Party, which supports Johnson's minority government in Parliament, believes a bridge would promote unity between the two countries.

DUP Transport spokesman Paul Girvan said in October last year: "As we leave the EU, the DUP has been clear that there should be no border erected down the Irish Sea. Instead of placing barriers between parts of the UK we should be building bridges."

"Government regularly commissions work to examine the feasibility of projects," a government representative told Channel 4. "During the leadership campaign candidates spoke about a number of issues which resulted in Number 10 commissions ahead of a new prime minister taking over. This PM has made no secret of his support for infrastructure projects that increase connectivity for people and particularly those that strengthen the Union."

Johnson has floated the idea of building bridges several times during his political career.

While he was mayor of London, he backed a campaign for a "Garden Bridge" to stretch across the Thames.

The project was beset by expensive problems and was eventually scrapped by his successor Sadiq Khan, but not before it had cost taxpayers £43 million.

He has also previously said he wanted to see a bridge across the English Channel to unite France and England, a route serviced by rail with the Channel Tunnel.