The UK is probing a near-miss involving a Stena ro-pax and a navy submarine.
The previously unreported "close quarters incident" took place on 6 November, the department for transport told TradeWinds.
The ship involved is the 30,300-gt Stena Superfast VII (built 2001), which operates between Belfast and Cairnyan.
The government is not releasing further details but said the question of how close the vessels came will be part of a full investigation by the Marine Accident Investigation Branch (MAIB).
An MAIB spokesperson said: “In November, we were notified of a close quarters incident between the ro-ro ferry, Stena Superfast VII, and a submarine operating at periscope depth.
"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 cooperation of the Royal Navy. A report will be published when our investigation has concluded.”
A spokesperson for Stena Line said: “Stena Line can confirm that on Tuesday 6th November 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 (MAIB) and we will of course cooperate fully in this."
