The Ministry of Defence has set up a board of inquiry to find out why a Devonport-based nuclear submarine collided with an object at sea.
HMS Tireless caused environmental protests in Gibraltar
HMS Tireless has docked at Faslane naval base in Scotland for repairs.
None of the crew was injured in the collision, and the Ministry insists there was no threat to nuclear safety at any time.
HMS Tireless was submerged when a ballast tank suffered minor damage, said a defence spokeswoman.
She would not comment on reports the vessel had struck an iceberg while in the Arctic area.
HMS Tireless also hit the headlines in 2000 when a crack appeared in a cooling pipe near its reactor and she was forced to berth in Gibraltar.
The MoD spokeswoman said: "She was submerged and hit a free floating object.
"There's a full assessment going on and she's going to Faslane for a full damage assessment.
"There were no injuries to the crew."
Faslane near Glasgow is the Royal Navy's base for Britain's Trident submarines.
HMS Tireless is one of seven Trafalgar Class submarines, based in Devonport, Plymouth.
The arrival of HMS Tireless in Gibraltar in 2000 led to protests by environmentalists concerned about radioactive leaks.
The crack led to all 12 of the navy's nuclear submarines being taken out of service for checks.