A BARROW-BUILT submarine has left Gibraltar after a collision with a shipping tanker and is understood to be returning to the UK for repairs.

The Royal Navy faces a repair bill likely to stretch to hundreds of thousands of pounds after HMS Ambush collided with a shipping tanker off the coast of Gibraltar at around 1.30pm local time last Wednesday.

The submarine's bridge fin struck a shipping tanker while it was surfacing during a training exercise, in an incident which has been described by the Ministry of Defence (MoD) as a "glancing collision".

Pictures show extensive damage to the fin, where some of the submarine's Thales sonar equipment is housed. A team from the shipyard in Barrow is on standby to fly out to assist with the repair work, although employees from BAE's Govan site in Glasgow may also be involved.

The Evening Mail understands HMS Ambush departed Gibraltar earlier today and this has since been confirmed by the MoD. The Gibraltar Chronicle quoted Spanish government officials who said the submarine was set to sail back to the UK for repairs.

Naval sources told the Evening Mail that initial reports based on photographs showing the damage suggested Ambush will not dive until at least a temporary fix is implemented. They added that the boat will not be able, or permitted, to dive if her sonar systems are damaged.

A former submariner who served in the Royal Navy for a number of years, told the Evening Mail it was a "hugely embarrassing" incident for the MoD and that it was highly unlikely to be anything other than human error.

"It won't be a mechanical or system error, it will be human error," the former submariner said.

Submarines operate a number of systems and procedures to prevent collisions and detection, including both active and passive sonar. A number of systems and alerts are in place to allow operators to detect nearby objects in extraordinary detail, ranging from other vessels to a shoal of fish.

When a submarine is surfacing, the boat will usually first rise to periscope depth, which is below the level at which any other vessel could be positioned. The periscope will then be used to ascertain the water above and around the vicinity of the submarine.

Additionally, all vessels belonging to countries which are part of the North Atlantic Treaty Organisation (NATO) are required to report their position to a central 'sea traffic control' to avoid such incidents. However, regardless of who was specifically to blame for the collision, in failing to detect and avoid the tanker, it is likely the sub's captain who will shoulder the blame.

"The buck will stop with the captain unless it was perhaps a case of gross negligence on the part of the operator," the submariner, who served on three Barrow-built boats, told the Evening Mail.

"The operator may not get that promotion he was after but he won't be sacked so to speak. The captain will be held responsible."

In a statement, the MoD said: "The submarine suffered some external damage but there is absolutely no damage to her nuclear plant and no member of the ship’s company was injured in the incident."

A spokesperson from BAE Systems Submarines in Barrow said: "We are aware of the incident involving HMS Ambush. We are in contact with the Royal Navy and stand ready to assist as required."