Prime Minister Boris Johnson has previously described the idea of a bridge between Scotland and Northern Ireland as being “very interesting”, with two different routes being considered by officials.

However, in an evidence session with Holyrood’s Europe committee, Mr Jack said that “the bridge for me is a euphemism for a link which is a tunnel”.

He added: “It would be less expensive to tunnel it. It’s no different to the tunnels connecting the Faroe Islands, it’s no different to the tunnels going under the fjords.

“Once we get a better sight of the costs involved, should the prime minister decide to press the button, we would then want to engage with both [the Northern Ireland assembly and the Scottish Parliament] to get a better understanding of the benefits and the challenges.”

Mr Jack said that there are “much longer tunnels in China”, and that a tunnel would deal with the issue of Beaufort’s Dyke - a dump of World War Two munitions - as well as “the problem of weather”, estimating a bridge might have to be closed for 100 days a year.

HE added: “The tunnel is at discussion stage. It may move to feasibility stage. In the interim we can start to have the debates about whether it’s viable, whether it’s going to help the Northern Ireland economy and the Scottish economy.

“Once we get better sight of the costs involved, should the prime minister decide to press the button, we would then want to engage with both Stormont and Holyrood to get a better understanding of the benefits and the challenges.

“We’re not going to just come riding roughshod and slam a tunnel in - and by the way under the settlement of devolution, nor can we.”