A major transport connectivity review is assessing the feasibility of a bridge or tunnel between Northern Ireland and Scotland.
The research is being carried out as part of Prime Minister Boris Johnson’s bid to improve UK transport links.
Network Rail chairman Sir Peter Hendy, who is carrying out a review of union connectivity, said further work is required on the possibility of a “fixed link” across the Irish Sea.
Following publication of his interim report, the Government announced that air passenger duty – a tax on passenger flights from UK airports – could be cut for domestic journeys.
Some £20 million has also been committed to develop plans for upgraded rail, road, sea and air links – and explore new requirements to offset emissions and decarbonise aviation.
Mr Hendy has commissioned two engineering professors to lead a study into the feasibility of a bridge or tunnel between Northern Ireland and Scotland, outlining its cost, timescale and the work involved.
They are ex-HS2 and Crossrail chairman Douglas Oakervee and former vice-president of Jacobs Engineering Gordon Masterton.
Mr Johnson has repeatedly spoken about the prospect of a bridge, even though experts have warned that the depth of the Irish Sea and the presence of dumped munitions would cause problems.
The scheme could cost a reported £20 billion, although the Prime Minister has previously said it would “only cost about £15 billion”.
The distance from Larne to Portpatrick, one of the most likely routes for a bridge, is around 28 miles (45km).
There is no problem with distance, money or the Beaufort’s Dyke explosives disposal area, according to Mr Johnson.
In November 2018, he said: “The problem is not the undersea Beaufort’s Dyke or lack of funds. The problem is an absence of political will.”
Transport Secretary Grant Shapps rejected a claim from Scottish First Minister Nicola Sturgeon that the money could be better spent elsewhere.
He told the BBC: “Why would you ever be against connecting different parts of our country in a better way? It shouldn’t be a controversial thought at all.”
Northern Ireland’s Infrastructure Minister Nichola Mallon accused the UK Government of “undermining devolution”.
“It’s very clear that all of this is about London being able to impose its decisions, determine its priorities for the people of Northern Ireland,” the SDLP minister told BBC Radio Ulster.
Conservative peer Lord Hannan told the BBC’s Politics Live: “I love the idea and I like the idea of strengthening the union and all the rest of it.
“But we just don’t have the cash.”
He added: “We’re going to have to start looking for ways to rein in spending across the board, including HS2, and I think not signing off on significant new infrastructure schemes.”
Mr Hendy’s interim report assessed ways transport can better connect all parts of the UK.
It set out how a UK Strategic Transport Network would deliver the ambition – upgrading direct transport links, reducing delays and stimulating growth across the four nations.
The Government’s £20 million will be spent on exploring the development of projects including improved rail connectivity between the north coast of Wales and England; upgrading the A75 between Gretna, Dumfries and Stranraer; faster rail links from England to Scotland, plus rail improvements in south-east Wales.
Mr Johnson said: “It’s now time to build back better in a way which brings every corner of the UK closer together.
“We will harness the incredible power of infrastructure to level up parts of our country that have too long been left off the transport map.
“This pioneering review by Sir Peter Hendy gives us the tools we need to deliver on our ambitions for a UK-wide transport network that encompasses sea, rail, and road – and I also want to cut passenger duty on domestic flights so we can support connectivity across the country.”