The review is led by former Network Rail chairman Sir Peter Hendy and was launched last year to look at strengthening transport connections between the UK’s four nations.

Responding to a call for evidence which closed at the end of December, industry body the High Speed Rail Group called for a railway tunnel connecting mainland Britain with Belfast to help address challenges in the post-Brexit Northern Ireland economy.

Among six further railway network improvements proposed by the group are an upgrade to the West Coast Mainline north of Crewe – after High Speed 2 – and conversion of the ‘Y’ shaped HS2 network to an ‘X’, in order to link Cardiff with Edinburgh.

High Speed Rail Group board member Jim Steer commented: “There is an urgent need for both new and improved transport links between the four nations of the United Kingdom,” which he said have been “neglected” for too long.

“Travel generates economic value, but the opportunity for further economic stimulus from this source will be lost if transport network capacity constraints are not addressed,” he added, calling for the group’s proposals to be taken up “as a matter of urgency”.

On the prospect of a rail tunnel linking Scotland and Northern Ireland, architect Professor Alan Dunlop – who first suggested creating a fixed crossing in the Irish Sea three years ago – said there appears to be “momentum gathering” on the idea.

However, he said: “I would like to see a rail and road link,” noting that both road hauliers and the railway sector have expressed support.

He added: “As far as iconography is concerned, a bridge would be better than a tunnel. The potential for such a connection is worth looking at; let’s have a serious debate about this.”

A further proposal by the High Speed Rail Group is to deliver a southern extension to the Borders Railway from Tweedbank down to Carlisle, which it said would strengthen connectivity in a region of poor roads and rail service.

Commenting, the Campaign for Borders Rail’s group chair Simon Walton described the scheme as “a missing link” that would add much needed capacity to the national network and ease pressure on the West Coast Mainline.

He added he is “quietly confident” that attitudes towards extending the Borders Railway have improved significantly in recent years and said the declaration of climate emergencies in both England and Scotland help to strengthen the case.

In addition the High Speed Rail Group proposed route electrification from Manchester Airport through North Wales to Holyhead on Anglesey, as well as upgrading railway from Newport to Crewe.

The Union Connectivity Review is expected to deliver final recommendations in the summer.

(Image: High Speed 2)