BORIS Johnson’s plan to turn the Isle of Man into a giant roundabout connecting a network of tunnels between Scotland, Ireland and England is “not credible”, the architect behind the Celtic Crossing has warned.

Over the weekend, it emerged that the Prime Minister was keen on a new plan to use the island to connect four roads from Stranraer, Belfast, Liverpool and Heysham in Lancashire.

A source told The Sunday Times that Johnson strongly believed in it: “Just as Hitler moved around imaginary armies in the dying days of the Third Reich, so the Number 10 policy unit is condemned to keep looking at this idea, which exists primarily in the mind of the Prime Minister.”

Number 10 has already ordered a feasibility study into a 25-mile crossing between Larne and Stranraer in Scotland.

Modelled on the Channel Tunnel, the so-called Boris Burrow would carry cars and lorries as well as rail passengers.

Network Rail chair Peter Hendy is due to publish his findings in the coming weeks.

One major problem for the new tunnel is Beaufort’s Dyke, a massive natural formation at the bottom of the North Channel.

Between the 1920s and 1970s, huge amounts of military waste were dumped into the dyke.

In 1997, the Government acknowledged that radioactive waste encased in steel drums had been dumped during the 1950s.

The idea for some form of bridge connecting the south west of Scotland with either County Antrim or County Down was originally put forward by Scottish architect Alan Dunlop in The National in January three years ago, arguing that it would be technically possible and that it would help to foster trading links between the two countries post-Brexit.

It was enthusiastically supported by the Prime Minister, though received a muted response from the Scottish Government.

Speaking to the Architect’s Journal, Dunlop said: “I cannot think why Johnson would say that, particularly at a time when there is so much criticism being levelled at the Prime Minister and the so-called Boris Burrow is being ridiculed in much of the press.

“It only serves to undermine that which I know to be structurally, technically and physically achievable, and that is a tunnel or bridge that connects Scotland with Ireland.”

Yesterday, one Number 10 source said the briefing of the tunnel plan was a sign of the recent dysfunction in Number 10.

“It’s one of those PM ideas that’s utterly mad ... and it’s the job of his team to stop it getting out and making him look silly. But instead it’s being briefed”.

MP Simon Hoare was sceptical. He said “the trains could be pulled by an inexhaustible herd of unicorns overseen by stern, officious dodos”.