A bridge or tunnel between Scotland and Northern Ireland would cost hundreds of billions of pounds, according to research commissioned by Boris Johnson.
The Prime Minister has previously talked up the creation of a fixed link to boost connectivity, but it is understood the idea has been scrapped as it would be too expensive and technically challenging.
A feasibility study led by Network Rail chairman Sir Peter Hendy found that a bridge would cost an estimated £335 billion, while a tunnel would be around £209 billion.
A bridge or tunnel would be the longest structure of their kind ever built.
It would take nearly 30 years to complete planning, design, parliamentary and legal processes, and construction, according to the research.
The report also said Beaufort’s Dyke – an underwater trench on the most direct route between Scotland and Northern Ireland – would need to be “carefully surveyed” due to a million tons of unexploded munitions being dumped there between the First World War and the 1970s.
A bridge would have a “sacrificial outer layer” enabling its main structure to survive a “local detonation”, the study said.
Sir Peter concluded it is “technically feasible to construct, maintain and operate” a tunnel or bridge but recommended that no further work should be carried out due to the cost.
One of the key recommendations of the report is to create a UKNet, which would map out strategic locations across the country and plot how to link them together, while providing extra funding for underperforming areas of the network.
The Prime Minister pledged to set up UKNet “right away”.
“If we want to truly level up the country then it’s vital that we improve connectivity between all corners of the UK, making it easier for more people to get to more places, more quickly,” he said.
“Sir Peter Hendy’s review is an inspiring vision for the future of transport which we will now consider carefully.
“Determined to get to work right away, we will set up a strategic UK-wide transport network that can better serve the whole country with stronger sea, rail and road links – not only bringing us closer together, but boosting jobs, prosperity and opportunity.”
Upgrades to the A75 in the south of Scotland were also recommended, which would improve connectivity to Northern Ireland, while calls were made for improving the A55, M53 and M56 and the South Wales Corridor in Wales, along with the North Wales Coast Main Line and rail links to the Midlands from Cardiff.
“My recommendations provide comprehensive, achievable and clear plans forward to better connect the whole of the United Kingdom, leading to more growth, jobs, housing and social cohesion,” Sir Peter said.
“I welcome the enthusiasm shown by the Prime Minister and the Government to my final report and I look forward to their formal response to my recommendations, which aim to spread opportunity and prosperity right across the United Kingdom.”
A Scottish Government spokesman said: “Transport is devolved to Holyrood and the UK Government should respect that.
“We will always seek to engage constructively with the UK Government – for example, on cross-border rail and our shared desire for HS2 to serve Scotland – but UK ministers have no role in deciding investment in Scotland’s trunk roads.
“Scottish ministers have not been sighted on the recommendations of the Union Connectivity report, however if UK ministers really want to play a helpful role, then they could simply deliver the funding we need for such infrastructure investment in line with established budgetary mechanisms for Scotland to determine our spending priorities.”