'Reactor safety case analysis' will be carried out on both submarines

HMS Torbay and HMS Tireless have joined three other submarines already undergoing refit at Devonport Dockyard.

It means just one of the nuclear submarines based there is currently fully operational.

The Royal Navy says that neither HMS Torbay nor HMS Tireless has failed any safety test and there have been no failures of plant or machinery.

Limited capacity

The Trafalgar Class nuclear powered submarines are among the Royal Navy's most sophisticated pieces of equipment.

Some of the boats are more than 20 years old and need regular maintenance and repair.

HMS Trafalgar was recalled for unscheduled repairs in October and is still undergoing sea trials.

HMS Torbay and HMS Tireless have been detained at Devonport for "reactor safety case analysis".

HMS Trenchant is the only fully operational nuclear submarine

That leaves HMS Trenchant the UK's only fully operational nuclear submarine at the moment.

Steve Bush, editor of Warship World magazine, says there is limited capability for refitting nuclear submarines.

He said: "Devonport is the main refitting base so there could be a log jam of submarines awaiting checks and inspections.

"That means operationally they can't get them all to sea at the moment."

A spokesman for the Royal Navy says safety is paramount when it comes to its nuclear submarines.

It is not yet clear when either submarine will be fully operational again.