HMS Torbay had already been rededicated for service

The problem is understood to involve a faulty control rod drive in the reactor.

Once the rods and metal fuel plates have been inserted, the reactor is sealed and the rest of the submarine's power system built around it.

The flaw was discovered only when the reactor was ready for final sea tests.

Independent nuclear expert John Large said: "It's egg on the face for the navy that such a problem has been discovered after the boat has been finished.

Hunter-killer

"Someone does a test and says one of the control rods isn't working, and they have to take it all to pieces again.

"It's not only HMS Torbay that is taken out of action - it also takes up that dry dock and shunts the entire refit programme back."

Devonport Dockyard is run by a private company

HMS Torbay was one of four Trafalgar class hunter-killer sumbarines found to have faults in their reactor cooling systems.

The same fault controversially stranded HMS Tireless in Gibraltar last year.

Mr Large said: "When HMS Tireless came up it affected not only all the other boats with the same problem, but every boat waiting for a refit.

"They all got knocked back."

He said the oversight was extraordinary.

Rigorous checks

"With nuclear power, every single action is completely supervised and scrutinised.

"If you changed a nut and bolt, there would be several checks.

"If a defective piece of kit has got through, particularly in the reactor, it's very embarrassing."

The refit began in 1998 and the rededication ceremony took place in June.

The Royal Navy admitted that the refit had been delayed, but insisted that HMS Torbay would return to active service this year.