BORIS Johnson is now exploring the idea of a multi-billion-pound rail tunnel connecting Scotland and Northern Ireland, it has emerged.

The Prime Minister, who has long floated the idea of a “Union Bridge” crossing the sea, has backed a review into a rail tunnel between Stranraer and Larne.

The chairman of Network Rail Sir Peter Hendy has asked experts to look into the idea.

The “Boris Bridge” proposals have consistently been dismissed by critics as impractical and a waste of money.

One expert said the project was “about as feasible as building a bridge to the moon”.

READ MORE: Boris Johnson’s Northern Ireland bridge plan to be reviewed

Concerns have been raised over any transport link having to avoid Beaufort’s Dyke, where tonnes of weapons were dumped in the sea between Scotland and Northern Ireland following the Second World War.

Meanwhile politicians in Scotland and Northern Ireland have said the billions of pounds needed for the project could be better spent on other vital infrastructure projects.

Hendy, who has been appointed by the PM to conduct a “Union connectivity review” into links between the four nations, said: “If you look at the distance between Northern Ireland and Scotland it is actually no further than the Channel Tunnel.

“I said to Boris, I am not going to get any further than finding out whether it is feasible, how long it will take and how much it might cost."

READ MORE: WATCH: Alister Jack says 'Boris Bridge' is a 'euphemism ... for a tunnel'

Last year Johnson asked civil servants to explore the building of a 21-mile bridge over the Irish Sea, which would cost around £20bn. But speaking to MSPs in March Scottish Secretary Alister Jack said the bridge was a “euphemism” for a tunnel.

Alongside the Beaufort’s Dyke considerations, it emerged Downing Street was concerned high winds could close a bridge for up to 100 days a year.

The new analysis into a potential rail tunnel will be led by Doug Oakervee, the author of a report which gave HS2 the green light this year.

Hendy told the Railway Industry Association annual conference: “The Government’s policy is to bring the United Kingdom closer together. The quest for economic growth, particularly in the light of Brexit, is a common desire for Westminster and for the devolved administration governments.

“If you look at air and ferry connections, one of the current bugbears for Northern Ireland is that since Flybe went bust there is much less opportunity to fly into Northern Ireland. They clearly find that very difficult. Maybe I can look at that and do something about it.”