Boris Johnson has signed off on a review to explore the possibility of building a rail tunnel between Scotland and Northern Ireland, despite criticism that the multi-billion pound venture would be impractical.

       The prime minister has repeatedly spoken in favour of building a fixed link between the countries, with Downing Street confirming earlier this year that the government was “looking at a wide range of schemes across the UK which could improve connectivity”.

       The chairman of Network Rail, Sir Peter Hendy, has asked experts to research the possibility of building a tunnel between Stranraer and Larne.

       He told the The Daily Telegraph: “If you look at the distance between Northern Ireland and Scotland it is actually no further than the Channel Tunnel.

       “I said to Boris, I am not going to get any further than finding out whether it is feasible, how long it will take and how much it might cost."

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       Significantly, any infrastructure built would have to avoid Beaufort’s Dyke, a natural trench between Northern Ireland and Scotland where tonnes of munitions were dumped in the aftermath of the Second World War.

       Earlier this year, Labour condemned the plan for a 21-mile bridge as “a distraction”, while one retired offshore engineer blasted the notion as being “about as feasible as building a bridge to the moon”.

       Alister Jack, the Scottish secretary, later told MSPs that said the bridge was a “euphemism” for a tunnel.

       Sir Peter told the Railway Industry Association annual conference: “The government’s policy is to bring the United Kingdom closer together. The quest for economic growth, particularly in the light of Brexit, is a common desire for Westminster and for the devolved administration governments."

       The review comes after Mr Johnson pledged ￡100m on 29 road projects during a speech in July on how to kick-start the economy following the coronavirus pandemic.

       Politicians from Scotland and Northern Ireland told the transport secretary in March that the multi-billion pound budget would be better spent on vital infrastructure projects.

       Michael Matheson, Scotland's transport secretary, had previously said the review has been "organised with virtually no consultation", despite transport being a devolved issue.

       He said: "We absolutely want to see improved transport and connectivity links beyond Scotland's borders – under any constitutional arrangements.

       "But this study is clearly part of the Tory government's wider agenda to undermine the devolution settlement across a whole range of policy areas."