The Trafalgar-class hunter-killer vessel has received an £80m upgrade to weapons and sonar systems as part of the work.

The new sonar equipment effectively doubles the range at which she can detect other vessels.

The Royal Navy said the upgrades make her one of the most sophisticated fighting machines in the world.

HMS Torbay has undergone sea trails

Commander Mark Cooper said: "It really is a huge update. It actually provides a huge increase in range of the submarine."

The refit, which began in 1998, suffered some setbacks when a faulty control rod drive in the reactor was discovered in August 2001.

The 4,750-ton vessel had already been given a rededication ceremony the preceding June, ready for a return to service, when the blunder emerged.

The fault should have been detected during rigorous tests that take place at every stage of such a refit.

DML, the private company that runs the Royal Navy dockyard, said the fault was investigated and fully amended before work proceeded.

The vessel is now ship-shape, according to her crew, who have just returned from sea trials off the United States following the refit.

HMS Torbay's sister-ships, Trafalgar and Triumph, were involved in the first attacks on Afghanistan in October 2001 which removed the Taleban from power.

In the New Year, the crew of HMS Torbay could be at the forefront of any conflict in Iraq.