Boris Johnson has shelved his dream for a bridge or tunnel connecting Northern Ireland with Scotland, after a review concluded it would be too technically challenging and expensive.
The Prime Minister tasked Sir Peter Hendy, the Network Rail chairman, with launching a feasibility study into the idea as part of a wider investigation into upgrading connectivity across the Union.
The review, expected to be published this coming week, has ruled out the proposal for a fixed-link connecting Northern Ireland to the rest of the UK for the foreseeable future.
A government source told The Telegraph: “Hendy has examined if this is affordable and practical and he concludes it would be technically very challenging at the moment.
“That’s not to say it won’t become viable at some point in the future, but at the moment it would be very, very difficult and expensive.”
The Prime Minister first tempered expectations about his proposal for a bridge or tunnel in September, telling reporters that while it remained an “ambition”, it was not “the most immediate” priority.
He served notice that if the idea ever went ahead, it would be “delivered substantially after the rest of the programme” for upgrading transport links across the UK.
It is understood that a bridge was ruled out by Sir Peter’s review first, amid concerns that the water in the North Channel of the Irish Sea is too deep to easily sink piles and piers – the “legs” of a bridge – into the seabed.
The proposed route would have crossed water that runs more than 1,000 ft deep in places, which would have required some of the largest support towers ever constructed. The area was also used as an offshore ammunition dump in the Second World War, which threw up additional problems.
Strong gales across the body of water meanwhile raised the prospect that the bridge would frequently be forced to close.
More time was spent investigating the feasibility of a tunnel, with input from two engineering professors. A sub-sea road tunnel was deemed a non-starter on account of its proposed length, because it would take too long for emergency responders to arrive if vehicles crashed in the middle.
A rail tunnel was examined in greater detail, with a route tunnelling under the Irish Sea between Stranraer and Larne submitted by the High-Speed Rail Group.
The principle challenge was posed by the fact that trains can only travel on gentle gradients, while the water running between the two territories is deep.
In order to avoid a steep descent, it was concluded that a tunnel would have to stretch a long distance and start far inland at both ends.
While an underwater rail link was not deemed impossible, it was viewed as complicated and prohibitively expensive in the medium term.
Sir Peter’s recommendations for boosting connections across the Union are expected to focus instead on road and rail links to Wales and to Scotland.
Mr Johnson has insisted the review will be “wonderful news” for all parts of the UK.
The Prime Minister has long harboured a dream to build a bridge between Northern Ireland and Scotland, first spelling out the idea three years ago.
However, the Chancellor is said to have been sceptical about the estimated multi-billion pound cost of the proposal, which critics argued would prove a white elephant. It did not feature in Rishi Sunak’s three-year spending review unveiled last month.