Boris Johnson's Irish Sea tunnel vision - roadworthy or pipe dream?

Dominic Cummings has described it as "the world's most stupid tunnel to Ireland".

Many others agree with the prime minister's former chief adviser, that Boris Johnson's tunnel vision is a bit of a joke, pie in the sky, a vanity project.

But another of Mr Johnson's former aides, Guto Harri, thinks that the idea shouldn't be dismissed entirely.

Mr Harri said Mr Johnson has "serious intent".

"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?" he said.

Mr Harri has been interviewed for Boris's Tunnel Vision, a documentary co-production for BBC Radio Ulster and Radio Scotland.

He was Boris Johnson's communications director for four years when the now prime minister was Mayor of London and he remembers how enthusiastic Mr Johnson was about a proposal to build a third London Airport in the Thames Estuary.

"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 said.

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

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

Just 12 miles separate Ireland and Scotland at the closest point - cultural and trade links go back centuries and ideas about a fixed link between the two countries have been put forward occasionally since the late 1800s.

It's a stormy stretch of water and there's the small matter of 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.

Nevertheless, Boris Johnson has asked Sir Peter Hendy, the Network Rail Chairman, to include the idea of an Irish Sea fixed link in his major review of the UK's road, rail and air infrastructure.

It's due to be published in the next few weeks.

Scottish architect, Prof Alan Dunlop, is one of the most enthusiastic supporters of the prime minister's plan.

He has drawn up his own design, modelled on the 650-mile Norwegian Coastal Highway, currently under construction, which will consist of a series of bridges and "floating tunnels".

Prof Dunlop's 'floating tunnel' would be connected and secured to the seabed by cables, similar to technology used on North Sea oil rigs.

He argues this would overcome challenges posed by the weather and the Beaufort Dyke, and that the tunnel would be a monumental engineering success as well as a vital economic and cultural link for two close neighbours.

It sounds far-fetched but tunnelling expert Mike Savill, of the international engineering company Mott MacDonald, said the idea would be worthy of serious consideration if the plan moves a step forward in the Hendy Review.

He told the BBC: "If you ask an engineer can you build it, he will always say yes - but in the end it will be the politicians and the transport planners who will decide. It will always be driven by political will."

However, political opinion in Scotland and Northern Ireland is sharply divided. Scottish Conservatives and Northern Ireland's Democratic Unionists are, not surprisingly, enthusiastic.

They argue that a tunnel would help cement their place in the union and boost trade and employment.

Galloway and West Dumfries Conservative MSP (member of the Scottish Parliament), Finlay Carson, said he supports the idea for political and economic reasons.

He said his priority was to see the A75 - the road linking Stranraer on the west coast to Gretna Green - improved but that he'd "welcome anything which might help improve links between the so-called Northern Powerhouse and Scotland's Central Belt, right to the coast and across the Irish Sea".

However, the Holyrood and Stormont transport ministers, Graeme Dey of the Scottish National Party (SNP) and Nichola Mallon of the Social Democratic and Labour Party (SDLP), are strongly opposed.

They believe it would be a colossal waste of money, which would be better spent on more pressing projects and, what's more, they see it as a Westminster power grab.

"This has been plucked out of thin air - there is no real substance to it," Mr Dey said.

"It's been deeply disrespectful to devolution, another example where Boris Johnston is stepping into the devolved sphere," said Ms Mallon.

East Antrim MP, Sammy Wilson of the DUP, said a tunnel would have to be a UK-wide project: "If you left it to the individual devolved administrations, nothing like this would ever be built, because their aims would never coincide.

"The Scottish Nationalists would have their priorities and we'll have ours and they'll tend to be local, because of course we'll be chasing after local votes."

Portpatrick, on the Galloway coast, was once a busy mail boat route to Donaghadee, in County Down.

Its hinterland is being suggested as a possible entrance/exit point for the tunnel, should it ever get the go-ahead.

Locals and visitors spoke to the programme about the idea of a tunnel connecting the devolved regions.

"I think it would be good," said one woman enjoying the sunshine and the view across the Irish Sea. "Scotland and Ireland are good neighbours aren't they?"

A passer-by whistles when told the projected cost of a tunnel could be £20bn.

"Whew - would that money not be better spent on the health service?" he said.

Another echoes Dominic Cummings' view: "I think it's the stupidest idea I've ever heard."

Guto Harri said that, with bigger priorities such as the National Health Service and social care reform, it is more likely, on balance, that the tunnel will not be built.

"But it is far from impossible," he added.

"The reality with Boris Johnson is that he could be prime minister for a long period of time.

"He could be out by Christmas, but if he is to be there for the best part of a decade, then this is an idea I think he would come back to. Then I think it more likely than not that this would get done."

Sir Peter Hendy's independent review is due to report soon. Perhaps then we'll find out more about the feasibility of Boris's Tunnel Vision under what the Ulster-Scots call 'The Narrow Sea'.

'Boris's Tunnel Vision' will be broadcast on BBC Radio Scotland on Sunday 22 August at 11:30 BST.

It will be broadcast on BBC Radio Ulster on Friday 27 August at 17:30 and repeated on Bank Holiday Monday 30 August at 12:30.

The programme is a Yamal Production, presented by Mark Simpson and produced by John Deering.