Scotland-Northern Ireland bridge: How likely is it to be built?

Another step has been taken towards the potential creation of a bridge or tunnel between Scotland and Northern Ireland.

An official feasibility study is to look at the possibility of some kind of "fixed link".

The research is being carried out as part of wider efforts to improve UK transport connectivity.

Two engineering experts will assess the scheme - along with outline costs and timescale and any associated work needed.

The idea has been around for some time with two potential routes suggested - from Portpatrick to Larne, or near Campbeltown to the Antrim coast.

However, are cost and technical issues likely to rule it out before it begins?

The price of any construction would, obviously, be dependent on the route chosen.

Sources have said the most likely option is between Portpatrick and Larne.

More than a decade ago the think tank, the Centre for Cross Border Studies, suggested a 21-mile bridge from Dumfries and Galloway could provide international rail links and ease the strain on air services.

At that time it estimated the cost of the scheme would be about £3.5bn.

However, the suggested price tag has risen considerably since then.

Some experts have suggested £15bn might be required for the project but others have said that £20bn would be a "conservative estimate".

In terms of distance - at more than 20 miles for the Portpatrick project - that bridge would not be the longest over water in the world.

That honour goes, according to Guinness World Records, to the Hong Kong-Zhuhai-Macau bridge which has 48.3km (about 30 miles) of its span over water.

It credits the 36km (22.4 mile) Hangzhou Bay Bridge as the structure spanning the greatest expanse of open sea.

They are all dwarfed by the longest bridge of all - the Danyang-Kunshan Grand Bridge - at 164 km (102 miles) which stretches the furthest but not over water.

According to a previous report by Channel 4, Prime Minister Boris Johnson wanted to know "the risks around the project" - including "WW2 munitions in the Irish Sea".

This is nothing new as concerns have long been expressed about the area known as Beaufort's Dyke - a deep submarine trench where it has been estimated that more than one million tonnes of weapons have been jettisoned.

Between logistics and costs, some have dismissed the scheme as unlikely to ever happen.

Last year Scotland's transport secretary Michael Matheson branded it a £20bn "vanity project".

However, one of the UK's leading architects Prof Alan Dunlop told Channel 4 News there would always be "naysayers".

"Such a project could be potentially tremendous for the country and show us as a forward looking and thinking country," he said.

He also previously told the BBC: "We do have incredibly talented architects and engineers in Scotland so I am sure that as a technical challenge it wouldn't be insurmountable."

It's understood one of the options being considered is a bridge which turns into a tunnel, before reverting to a bridge.

Plans for some kind of link - either a tunnel or a bridge - go back as far as the 1890s but, despite much consideration, nothing has ever been built.

However, some political will has emerged in recent years.

In Northern Ireland, a proposal was contained in the 2015 DUP manifesto.

One MP told the BBC a bridge could make travel "less expensive and probably more certain".

Dumfries and Galloway Council said it was "certainly not averse" to the proposal being examined but felt there were other "more immediate priorities" for investment.

In particular, it highlighted the trunk roads network from the Cairnryan ports to the motorway network.

The Scottish government has said that any such project would require close examination before it could even be considered.

"We are always keen to talk about how we can strengthen connections between Scotland, Northern Ireland and the Republic of Ireland," a spokesman said.

"There are an obvious number of practical obstacles and challenges to such a concept, and it would require a robust assessment of the costs or benefits of such a project in the first instance."

The UK government has now said that it will look at the feasibility of a bridge or tunnel as part of a wider review of transport links.

A report is expected back in the summer.