BRITAIN’S two new aircraft carriers could cost £5 billion to build — which is £2 billion more than the Ministry of Defence’s original estimate.

Plans to shrink the ships and come closer to the MoD’s £2.9 billion budget have been shelved. The final outline design calls for “full-size” carriers about 280m long, weighing 65,000 tonnes and capable of handling 40 to 45 aircraft.

BAE Systems and Thales, the defence contractors which last year won a contract to design the ships in partnership with the MoD, are expected to be awarded a follow-on contract in late March or April to further refine the design — and reduce its cost.

The two companies last night declined to comment on the state of the negotiations with the MoD, or the cost of the vessels. But suppliers to the programme say that a final construction bill of £5 billion or more is now expected.

One source close to the talks said the United States had allocated £6.6 billion for the design and construction of its latest aircraft carrier, the CVN21. “Admittedly, that is a larger vessel and is nuclear-powered, but it’s just one ship and we are building two,” he said.

These disclosures about the size and cost of the vessels will reawaken the debate over the reliability of the original budget estimates. “It was always clear that the £2.9 billion figure was just not realistic,” the source added.

The carrier design has gone through three distinct phases. When the BAE-Thales alliance was awarded a contract, construction of a 65,000-tonne ship was envisaged.

Designers then tried to shrink the vessels to come closer to the target cost. Plans for a 55,000-tonne, 260m ship were drawn up but, after consultation with the MoD and the Defence Procurement Agency, it was decided that the complexities of the smaller design made it too risky. “If you wanted the ships to be just as capable, but smaller, it meant you had to be more innovative in some areas of design and construction, and it was felt that just brought more risk than was necessary,” an insider said.

The alliance has now settled on an “equilibrium” solution. Costs have already been reduced by scaling back some of the ship’s defensive systems. “The reality is that these ships will never leave port without being in a taskforce, including submarines and destroyers to protect them,” said the official.

Negotiations have now begun on an extension of the alliance approach used so far on the programme. It is understood that this could develop into a commercial agreement between British shipyards that would see the carrier work shared between them.

The April contract will be for a further 12 to 18-month refinement of the design, aimed at achieving certainty on construction costs. Despite suggestions that the programme is now running several months late, sources close to the project insist that it remains on track.

The French government’s requirement for another aircraft carrier could yet intrude into the UK’s plans.

Some Whitehall sources believe that the two countries could yet collaborate on the construction of the warships, which would see some work going to French shipyards.