BORIS Johnson’s plan to build an undersea link between Scotland and Northern Ireland would be the “world's most stupid tunnel”, his former chief aide has said.

Dominic Cummings said the idea was typical of the Prime Minister’s scatterbrained thinking.

He said: “The Prime Minister’s only agenda is, buy more trains, buy more buses, have more bikes, and build the world’s most stupid tunnel to Ireland. That’s it.”

As well as being embarrassing for the PM, the remarks are acutely awkward for Scottish Secretary Alister Jack, who has described the Union Tunnel idea as “fantastic”.

In March, a Union Connectivity Review ordered by Mr Johnson into more transport links between the four UK nations recommended a feasibility study into a tunnel connecting Great Britain and Northern Ireland, which critics swiftly denounced it as a “vanity project”.

The 45-km (28 mile) tunnel from Portpatrick in Dumfries & Galloway to Larne in County Antrim, dubbed the ‘Boris burrow’, would cost £15bn to £20bn.

READ MORE: Experts to look at feasibility of building £20bn 'Union Tunnel' between Scotland and N Ireland

Both the Scottish and Northern Ireland governments have questioned whether the money would be far better spent on something else.

Mr Johnson has championed the idea since before becoming Prime Minister in 2019.

The year before he replaced Theresa May in Downing Street, he said: “The problem is not the undersea Beaufort’s Dyke [a deep water munitions dump] or lack of funds. The problem is an absence of political will.”

Mr Cummings made the comments in an interview with BBC Political Editor Laura Kuenssberg.

In it, he repeatedly discussed pulling Mr Johnson’s strings and manipulating the political direction of the country in Downing Street, despite never having been elected. 

He insisted he was trying to deliver the best outcome for the nation.

“The sooner he goes the better,” he said of Mr Johnson, citing the interview itself as a contribution to the effort.

Talking about the breakdown of the pair’s relationship late last year, Mr Cummings did not deny that Mr Johnson's perception that he was a “puppet” of the Vote Leave team Mr Cummings brought with him to Downing Street. 

He said the PM had been angered by that view of him, by differences over how to handled thhe pandemic, and by Mr Cummings ploughing his own furrow on Whitehalll reform, defence and science and technology.

READ MORE: Jack: Scottish Government 'pathetic' for not engaging in Union Connectivity transport review

Mr Cummmings said: “I had a plan. I was trying to get things done. He didn’t have a plan. He didn’t have an agenda.

“The Prime Minister’s only agenda is, buy more trains, buy more buses, have more bikes, and build the world’s most stupid tunnel to Ireland. That’s it.”

Mr Cummings was eventually fired last November, as relations between the men and between Mr Cummings and Mr Johnson's then fiancee, Carrie Symonds, deteriorated.