The Astute-class submarine, which should already be in service, will not be available for operations until 2009.

But the latest details to emerge of this new-generation submarine show that she will dwarf the Navy’s existing fleet of “hunter-killer” boats.

The finished hull of the 7,800-tonne HMS Astute, the first in the class of three ordered so far at a cost of £3.5 billion, is more than 318ft (97m) long, nearly 37ft wide and more than 52ft high.

The ageing Swiftsure-class nuclear-powered attack submarines, by comparison, are about 280ft long and 32ft wide, and have a displacement of only 4,200 tonnes.

The construction of the Astute-class submarine is in line with the Navy’s new policy of building significantly bigger platforms for future operations: the two planned aircraft carriers will be 60,000 tonnes, three times the size of the existing Invincible-class carriers; the new Type 45 destroyer is 8,000 tonnes, compared with the 3,880-tonne Type 42 destroyers that it is replacing.

The Navy is hoping to have eight Astute-class submarines, which will replace all the Swiftsure boats and some of the Trafalgar-class boats. Under present plans, the remaining Swiftsure boats will go over the next few years, and the seven newer Trafalgar-class submarines will be taken out of service in a phased programme starting in 2008 and finishing in 2022 with the decommissioning of HMS Triumph.

The expectation is that the Astute-class submarines will remain in service for 25 years, performing a variety of roles that will fit into the Government’s strategy of expeditionary warfare.

The concept of submarines hunting the oceans for enemy boats vanished with the ending of the Cold War. Now the primary role of the underwater machines is supporting land operations.

The Astute-class boats will be equipped with Tomahawk land-attack cruise missiles and a dry dock for delivering special forces on secret missions. Their other significant role, like all attack submarines, will be to protect Britain’s nuclear ballistic missile deterrent carried by the four Vanguard-class submarines.

Pictures of HMS Astute under construction were released by the MoD and BAE Systems, which has the contract to build the three submarines at Barrow-in-Furness, in Cumbria.

Nearly £1 billion has been added to the cost of the programme because of initial management and design problems. The MoD was forced to renegotiate the original contract. The new in-service date for HMS Astute, which moved from 2006 to 2008, was quietly slipped to 2009 this year.

The next two boats, also under construction at Barrow, are HMS Ambush and HMS Artful. All the Astute-class vessels are being equipped with a nuclear pressurised water reactor that will last, without needing to be refuelled, for their full service life.

They will also be the first submarines to go on patrol without the traditional periscope, which is being replaced by thermal imaging cameras and high-definition television sensors attached to masts. The submarines will still have to come up to the equivalent of “periscope depth” for the cameras to have a quick 360-degree look around.

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