Britain launches massive submarine that can hear a ship from across the Atlantic

She is four years late and a massive £900million over-budget.

But when the Royal Navy's super-sub HMS Astute finally arrived, she made for an awesome sight.

More complex than the space shuttle, and able to circumnavigate the globe without surfacing, the 7,400-ton monster is the largest and deadliest hunter-killer submarine ever built.

The Duchess of Cornwall cracked a bottle of beer – brewed by the sub's crew – on her prow to officially name the "boat", in Navy jargon, before she was gingerly wheeled out of her shed at the stately speed of one metre per minute.

The specifications for Britain's biggest submarine make for mind- boggling reading, but it was the sheer size of the black behemoth which made its mark on the 10,000 dockyard workers, schoolchildren, VIPs and Navy personnel invited to the ceremony in Barrow-in-Furness, Cumbria.

As long as a football pitch, at 318ft, and as wide as four double- decker buses, HMS Astute is a third longer than any sub which has gone before.

Her nuclear-powered engine will propel her through the water at more than 20 knots, yet the UK's first stealth sub makes less noise than a baby dolphin, making her as good as undetectable by enemy ships.

Astute's sonar is so advanced that if she was lying in the English Channel she would be able to detect ships leaving New York harbour 3,000 nautical miles away (although the details of how she can do this are classified).

The nuclear reactor will never need refuelling, and with an ability to make oxygen and drinking water out of sea water, the sub could stay underwater for its entire 25-year lifespan were it not for the needs of the crew.

Once she goes into operation in 2009, Astute will carry a 98-man crew and stay at sea for 12 weeks on a routine patrol.

She will carry 38 Tomahawk cruise missiles, with a range of 1,240 miles, meaning Astute could attack targets in North Africa with pinpoint accuracy while sitting off the coast of Plymouth.

Spearfish torpedoes will also be on board for attacking ships and other subs.

But Astute will not carry nuclear weapons – the UK's Trident missiles are launched from the Vanguard class of submarines.

The Navy's submarine chief Captain Mike Davis-Marks said: "The Astute class of submarines will quite simply be unbeatable worldwide for many years to come.

"Astute will have a capability that will keep us right at the top of the premiership of the world's navies – the Manchester United of submarine nations. With our proud heritage, Britain deserves nothing less."

Astute is the first of four vessels to be built by BAE Systems at a total cost of £3.85billion, or £960million each.

She will be joined by HMS Ambush, HMS Artful and HMS Audacious, with an option for a further three subs to come.

Defence procurement minister Lord Drayson called Astute "a truly remarkable vessel" whose importance "cannot be over-estimated".

As the Duchess of Cornwall named the sub – the first such ceremony she has carried out – she said: "As an admiral's wife myself, I am delighted to be in Barrow-in-Furness today for the naming and launching of Astute. I shall follow her progress with particular interest and hope to see her in the near future."

She was presented with a retriever puppy, similar to one which appears on the Astute's crest, which will be donated to Guide Dogs for the Blind.

The boat, which will replace the Swiftsure and Trafalgar classes, will be gently lowered into the water. It will eventually be based in Faslane on the Clyde in Scotland.