By Lucy Rodgers BBC News

HMS Tireless can deal with extreme environments (Picture: Royal Navy)

They may be rarely seen or heard, but Britain's secret, underwater hunters play a key role in the country's defence.

The Royal Navy's fleet submarines provide essential support to Britain's four Vanguard submarines, which are armed with strategic ballistic Trident missiles.

But the seven Trafalgar class and two Swiftsure class vessels are themselves sophisticated weapons tasked with seeking out and destroying enemy submarines that may pose a threat.

However, despite their military sophistication, safety on board the fleet subs is now under scrutiny after an explosion on HMS Tireless killed two British sailors.

The third Trafalgar class vessel in the fleet, based at Devonport, Plymouth, was taking part in a joint British-US operation under the Arctic icecap when an emergency oxygen device malfunctioned on Wednesday.

TRAFALGAR CLASS Crew - 130 Main weapon - Tomahawk cruise missiles Speed - 36.8mph (32 knots) Length - 85.4m (280ft) Width - 9.8m (32.2ft)

The MoD has insisted the equipment on the 23-year-old submarine had a 100% safety record and that the ship itself had not been in danger.

Primarily designed for a Cold War role, the Trafalgar class has since taken part in the Kosovo conflict and attacks against the Taleban in Afghanistan.

The submarines also play an important part in intelligence-gathering, because of their ability to remain undetected.

Mike Critchley, an ex-Royal Navy officer and publisher of Warship World magazine, said: "Their role is to essentially take out the enemy, but they have a major intelligence role - listening, taking pictures."

Wednesday's accident was not the first time HMS Tireless has been at the centre of controversy.

In November 2000, 12 Royal Navy fleet submarines were taken out of service after a crack was discovered in HMS Tireless' cooling system.

There have been two vessels called HMS Tireless (Picture: Royal Navy)

The vessel had limped into Gibraltar in May of that year, with a leak in a cooling pipe near its nuclear reactor, sparking fears that it could leak and contaminate the environment.

Its year-long docking at the UK base on the Rock for repairs caused outrage among environmentalists and strained relations between Spain and the UK.

Then in 2001 it was revealed that a programme for upgrading all of the Royal Navy's Devon-based fleet was one year behind schedule and �68m over budget.

And in May 2003 the vessel crashed into an object, which may have been an iceberg, while on patrol in the Arctic and suffered minor damage to the ballast tank.

But Mr Critchley said a submarine experienced mechanical and technical problems just like a car.

SWIFTSURE CLASS Crew - 116 Main weapon - Tomahawk cruise missiles Speed - 23mph (20 knots) Length - 82.9m (272ft) Width - 9.8m (32.2ft)

"If your gearbox goes, you take it to a garage and get it fixed and keep driving it," he said. "[HMS Tireless] was fixed and went back into operation."

HMS Tireless, in name at least, has enjoyed a more celebrated past.

The first submarine to bear the Tireless name was authorised under the 1941 War Emergency Program, joining the British Pacific Fleet and arriving in Hong Kong in 1945.

Propelled by diesel engines and electric motors, the vessel went on to take part in exercises in the Far East and made visits to Australia and Japan, before returning to the UK for a refit.

Specialist capabilities

After World War II, HMS Tireless joined the Second and Third Submarine Flotillas, visiting many of the major European ports as well as carrying out exercises with Nato.

But 10 years after appearing in the 1953 Coronation review of the fleet, the vessel was finally sold to a breakers yard.

The latest HMS Tireless, armed with five tubes capable of firing Tomahawk missiles, has completed a number of patrols from the North Atlantic to the Mediterranean, including several successful under-ice expeditions.

And Mr Critchley said the ship must have specialist ice capabilities allowing it to complete intelligence-gathering patrols in extreme environments.

"The last sub to be sent to the Arctic was HMS Tireless - so she probably has some technical capabilities that other subs do not, allowing her to function there better."