BORIS Johnson is “serious” about building a tunnel or bridge linking Scotland and Northern Ireland, his former aide has said.

Guto Harri, who was Johnson’s communications director for four years when he was mayor of London, said it was part of the Prime Minister's plan to “maintain the Union”.

The comments were made by Harri during an interview for Boris’s Tunnel Vision – a documentary co-produced by BBC Radio Ulster and Radio Scotland.

Dominic Cummings once described the plans as “the world’s most stupid tunnel to Ireland”.

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If the project goes ahead it could cost a staggering £33 billion, according to former Transport Secretary Michael Matheson, who also predicted it would not happen in the PM’s lifetime.

And now, Harri has said that Johnson has “serious intent” and that the idea shouldn’t be dismissed entirely. He said: "He wants to maintain the Union and he wants to persuade the rest of the world that we're still big players and can build big things – and what would be bigger than a tunnel under the Irish Sea, linking Scotland and Northern Ireland?"

There are only 12 miles between Scotland and Ireland at the closest point, a proposed plan for the bridge would put the structure between Portpatrick in Wigtownshire, Scotland, and Larne in County Antrim, Northern Ireland.

However, in the stretch of water that is being considered there are thousands of tonnes of World War Two munitions dumped in the Beaufort Dyke, a deep trench which runs down the middle of the Irish Sea. It also contains radioactive waste.

The National:

Johnson's former aide said he had "serious intent" to build the bridge between Scotland and NI

Despite this, Johnson asked Sir Peter Hendy, Chairman of Network Rail, to include the idea of a fixed link in the Irish Sea in his major review of the UK’s road, rail and air infrastructure.

It is understood the review is set to be published in the next few weeks.

We previously told how Scottish Architect Professor Alan Dunlop released drawings of the proposed multi-million pound bridge, based on the Oresund bridge which links Sweden and Denmark.

Dunlop’s plans show a long road link from the north coast of Ireland to Campeltown in Argyll.

It wasn’t the only bizarre idea of Johnson’s that Harri discussed on the documentary, as he explained how Johnson reacted to a proposal to build a third London Airport in the Thames Estuary.

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Harri said: "I remember him ringing up a Tory MP in the House of Commons and singing Dolly Parton's 'Islands in the Stream, that is what we are' down the phone to him.

"He made it look as if it was a joke, but he was very serious."

Harri said that in the end, a lack of finance and authority forced Johnson to abandon his floating airport project.

Earlier this year, Social Democratic and Labour Party (SDLP) leader Colum Eastwood challenged Johnson over the bridge proposals at PMQs.

Eastwood dubbed the bridge a “fantasy” and hit out at the plans while the country’s “road and rail networks have been absolutely decimated from decades of under investment”.

And, one expert previously described the bridge as “about as feasible as building a bridge to the moon”.