A study commissioned by Boris Johnson examining the idea of a bridge or tunnel between Scotland and Northern Ireland cost taxpayers almost £900,000.
Research into the possibility of a fixed link across the Irish Sea cost £896,681, the Department for Transport said.
Network Rail chairman Sir Peter Hendy led the exercise, which concluded it would be technically feasible but “impossible to justify” as the “benefits could not possibly outweigh the costs”.
The study found a bridge would cost an estimated £335bn, while a tunnel would be around £209bn.- despite the prime minister once suggesting it could be done for “about £15bn”.
It said the work required would be incredibly challenging, taking nearly 30 years from planning to completion.
Beaufort’s Dyke – an underwater trench on the most direct route – would also have to be “carefully surveyed”, the study said, due to about a million tons of unexploded munitions dumped there between the First World War and the 1970s.
A bridge was described as needing a “sacrificial outer layer” enabling its main structure to survive a “local detonation”.
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The Oresund Bridge, a 10 mile-long road and rail link between Sweden and Denmark, was viewed as an inspiration for the potential UK crossing.
Mr Johnson had previously been enthusiastic about the possibility of a link but accepted the report’s conclusion.
The study was carried out in tandem with a wider review of UK connectivity which cost another £1.1m.
Sir Peter led the review alongside his Network Rail job and did not get any extra pay.