The comment came after the industry lobbyists voiced their support for the tunnel as part of the UK Government’s Union Connectivity Review to improve links between the four nations of the UK.
Sustainable transport campaigners Transform Scotland also today said there was “no evidence case on grounds of travel demand for a fixed link between Scotland and Northern Ireland” and called for improved public transport access to the Cairnryan ferry terminal instead.
The HSRG said: ”Provision of a cross-Irish Sea rail tunnel with connecting rail links to Carlisle and Belfast binds Northern Ireland closer to Great Britain and helps address challenges in the post-Brexit Northern Ireland economy, as well as increasing connectivity for south west Scotland.”
It is among seven “key improvements” proposed by the HSRG to the review along with completing the re-opening of the Waverley line between Edinburgh and Carlisle, and extending high-speed rail to Scotland.
The group said: “While a cross-Irish Sea tunnel would be costly, such considerations have been overcome in exciting and transformational infrastructure schemes in north west Europe, most notably in the Faroes Islands and in Norway and between Denmark and Germany.”
It said the Channel Tunnel could be used as a model, and a 1901 proposal for a tunnel would avoid the Beaufort’s Dyke underwater trench where wartime bombs were subsequently dumped.
The HSRG said its proposals “would in turn strengthen the Union between the four nations of the United Kingdom”.
However, a spokesperson insisted that was not a comment on Scottish independence.
They said: “Irrespective of the political future of the British Isles, stronger transport links will strengthen us socially, culturally and economically.
"That’s got to be in everyone’s interests.”
However, South Scotland SNP MSP Emma Harper said: "My first rail preference would be to re-establish the original rail links across the south west and to connect the Borders Railway with Carlisle rather than invest in a fantasy tunnel across an unexploded ammunition dump.
"It has been four years since the Tories promised three hour journeys between Scotland and London.
"I think they should focus on that first rather than blue-sky ideas like a tunnel."
Transform director Colin Howden said: “There is simply no case in terms of travel demand for a fixed link across the Irish Sea.
"There has actually been a decline in the overall travel market between Scotland and Northern Ireland over the past decade.
“Compared to Anglo-Scottish travel, where aviation provides the bulk of passenger travel and [lorries] most goods traffic, transport from Scotland across the Irish Sea involves the comparatively more sustainable modes of ferries and shipping.
“What is required is the improvement of public transport links to the port of Cairnryan, so that more passenger journeys to Northern Ireland can be made entirely by sustainable modes.”
The Scottish Government has refused to take part in the connectivity review, accusing the UK Government of a “blatant power grab”.
Scottish ministers are undertaking their own Strategic Transport Projects Review to decide on the country’s next big projects.
However, Scotland Office minister Iain Stewart warned today the SNP not engaging with the connectivity review “could prevent billions of pounds of investment and jobs reaching Scotland”.