A FERRY crossing between Scotland and Northern Ireland had to avoid a nuclear submarine, an official probe has heard.

Although there was no collision, it was the third accident or incident between a dived Royal Navy submarine and a surface vessel in four years.

Investigators said it was 'a matter of significant concern'.

The Faslane-based submarine, which has not been named, was said to be on a training exercise with 282 people on board.

The ferry had 215 passengers and 67 crew, as well as cars and freight.

It was at 12.56pm on November 6 2018, that the Stena Superfast VII's officer of the watch took action to avoid collision with a submerged submarine that had been spotted at close range ahead of the ferry.

The 667 ft long Stena Superfast VII was on a scheduled North Channel crossing from Belfast to Cairnryan in Wigtownshire.

The Royal Navy submarine was at periscope depth conducting pre-deployment safety training in the same vicinity.

The submarine's command team detected and tracked the ferry using visual, sonar and automatic information system data. As the ferry's range reduced, the submarine's officer of the watch altered course to avoid it.

However, this turn was towards the ferry and reduced the time available for the submarine to keep out of the ferry's way, said the Marine Accident Investigatgion Branch report.

With the sonar contact on a steady bearing, the submarine's sonar team initiated a close quarters procedure - the commanding officer was also called to the control room.

Based on the picture displayed by the submarine's electronic tactical command system, the commanding officer intervened to cancel the close quarters move and ordered that the submarine remain at periscope depth rather than go deep to its safe depth.

At about the same time, Stena Superfast VII's lookout spotted a submarine periscope close on the port bow, and alerted the officer of the watch, who took immediate action to avoid collision.

After taking avoiding action, the ferry's closest point of approach with the submarine was about 250 yards, which was unsafe, said the MAIB.

However, the submarine's commanding officer believed the passing distance to be about 1000 yards, or four times the actual range.

“This incident happened because the submarine’s control room team overestimated the ferry’s range and underestimated its speed. This combination meant that the submarine’s commanding officer and its officer of the watch made safety-critical decisions that might have appeared rational to them at the time but were actually based on inaccurate information,” said the MAIB.

“Two previous collisions between Royal Navy submarines and surface vessels show a similarity in that key decisions on board the submarines were made based on an insufficient appreciation of the location of surface ships in the vicinity. The Royal Navy has taken a series of actions in response to this incident, and previous similar accidents.”

As a result, the report made a safety recommendation to the Royal Navy to undertake an independent review of its actions taken to ensure that such actions have been effective in reducing the risk of collision between dived submarines and surface vessels.

Andrew Moll, Chief Inspector of Marine Accidents said: “The officer of the watch then took immediate and effective action, turning the ferry to avoid a genuine risk of collision with a submerged submarine. The incident happened because the submarine’s control room team had underestimated the ferry’s speed and overestimated its range, resulting in safety-critical decisions being made based on inaccurate information.

“Although there was no collision, this was the third accident or incident between a dived Royal Navy submarine and a surface vessel in four years, which is a matter of significant concern. The Royal Navy co-operated with the MAIB’s investigation into this near miss and has taken a series of actions, intended to prevent recurrence, in response to this and the other similar incidents.

"However, I have today recommended that the Royal Navy undertakes an independent review of the actions that have been taken, in order to ensure that the risk of similar collisions has been reduced to as low as possible.”