SINN Féin has called for the Irish Sea to become a "submarine free zone" after it emerged that a nuclear-powered Royal Navy submarine had a near-miss with a ferry.
South Down MP Chris Hazzard said the incident had "raised serious questions" and said "lives of those on board the ferry were put at risk".
It comes after the Marine Accident Investigation Branch (MAIB) confirmed it had launched an investigation into the near-miss involving the Stena Superfast VII and a submarine in the Irish Sea on November 6.
They are examining reports the ferry, which operates between Northern Ireland and Scotland and has the capacity for 1,300 passengers, was in "a close-quarters incident" with a "submarine operating at periscope depth".
A MAIB spokesman said: "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."
The Royal Navy would not confirm which of its 10 submarines was involved. They are all nuclear-powered but only four carry Trident nuclear missiles.
A Royal Navy spokesman would only "confirm the sighting" of a Royal Navy submarine between Belfast and Stranraer, adding that it is "co-operating with the MAIB's investigation".
A Stena Line spokesman also confirmed its ferry "came into close proximity" with a submerged submarine during a scheduled crossing between Cairnryan and Belfast.
"At no stage were the vessel, passengers or crew in any danger," he added.
But Mr Hazzard said the incident highlighted the need for the Irish Sea to become a "submarine free zone".
"The confirmation by the British Ministry of Defence that a Royal Navy submarine was involved in a near-miss with a ferry in the Irish Sea has raised serious questions," he said.
"The lives of those on board the ferry were put at risk.
"The British government and the British Ministry of Defence should end its submarine activity in the Irish Sea to avoid similar incidents in the future."
