The fantasy of building a bridge that connects mainland UK with Northern Ireland is laid bare in a report judging the feasibility of the project, which found it would cost £335bn and need bombproof foundations.

Released alongside a wider review calling for investment in roads and to better connect the four UK nations, the fixed link report found either a bridge or tunnel was almost beyond what was currently possible with the available technology.

As a result, the government admitted that the idea for a structure connecting Larne in Ulster with Portpatrick in Galloway and mooted by prime minister Boris Johnson, had been abandoned.

The review acknowledged that freight connectivity was an essential part of the successful operation of efficient supply chains and delivering economic growth.

Among its recommendations were upgrading the A75 link to improve freight and passenger connectivity with Northern Ireland; improvements to the A55, M53 and M56 and relieving congestion on the M4 South Wales and England corridor and easing capacity restrictions at the junction of the M4, M5 and M32.

Elizabeth de Jong, policy director at Logistics UK, said: “With effective and efficient transport connectivity between the nations vital to support trade and the UK economy, Logistics UK welcomes the infrastructure improvements recommended in the review, including UKNET, a strategic transport network that would span the UK, and the A75 upgrade.

“The A75 in Scotland is a key trading route linking Great Britain and Northern Ireland, but the road has been in need of investment for many years.”