A nuclear-powered Royal Navy submarine had a near-miss with a ferry sailing between Northern Ireland and Scotland, it has emerged.

An investigation has been launched into the incident in the Irish Sea involving the Stena Superfast VI, which runs between Cairnryan and Belfast.

The MoD has not confirmed which of its 10 submarines was involved but all the vessels only four carry Trident nuclear missiles.

The MoD insisted there was no danger from the November 6 incident to the vessel, which can hold 1,300 passengers and 660 cars.

A Marine Accident Investigation Branch (MAIB) spokesman said they were notified of a "close-quarters incident" involving the roll-on-roll-off ferry and a submarine "operating at periscope depth".

The spokesman added: "We have carried out a preliminary assessment of the evidence in this case and the Chief Inspector of Marine Accidents has decided to open a safety investigation.

"The investigation is being conducted with the full co-operation of the Royal Navy. A report will be published when our investigation has concluded."

A Royal Navy spokesman said: "We can confirm the sighting of a Royal Navy submarine between Belfast and Stranraer on 6 November 2018. We are co-operating with the MAIB's investigation."

A spokesman for Stena said: "Stena Line can confirm that on Tuesday, November 6, 2018, Stena Superfast VII and a submerged submarine came into close proximity during a scheduled crossing between Cairnryan and Belfast.

"At no stage were the vessel, passengers or crew in any danger. The incident is now under investigation by the Marine Accident Investigation Branch and we will of course co-operate fully in this."