They were meant to be the pride and joy of Britain’s 21st-century fleet, an emblem of its pre-eminence as a military and diplomatic power. But last night it emerged that the imminent announcement of the commissioning of two aircraft carriers at a total cost of £4 billion has left some of the country’s senior military leaders deeply unhappy.

General Sir Timothy Granville-Chapman, the Vice-Chief of the Defence Staff, has written confidentially to all one-star and two-star officers in the Ministry of Defence — equivalent to brigadier and major-general — asking for their views about the need for a next-generation carrier strike force.

Next week the Government is expected to commission two 65,000-tonne aircraft carriers, creating thousands of jobs in the process but putting other military budgets under enormous pressure.

Although the three Service chiefs are all signed up for the carrier programme, there remain doubts and concerns inside and outside the MoD about the wisdom of building two ships that will be more than three times the size of the present Invincible-class carriers.

The last-gasp canvassing of opinion from senior officers of all three Armed Forces working at the MoD also reflects concern, particularly in the RAF, over threatened cuts in other equipment procurement programmes to help to pay for the carriers. There is increasing speculation that the RAF’s Super Lynx helicopter project and the third phase of the Eurofighter/ Typhoon programme may have to be scrapped.

The Government has been committed to the construction of two larger aircraft carriers since 1998, when they formed the heart of a new expeditionary policy for the Armed Forces, outlined in the Strategic Defence Review. But the small print revealed the plan was to have two 40,000-tonne ships, only twice the size of the Invincible-class carriers. The estimated cost at that time was £750 million. But the tonnage has crept up to 65,000 tonnes.

Andrew Brookes, of the International Institute for Strategic Studies, said: “The MoD simply can’t afford the carriers. There’s a black hole in the defence budget which has amounted to about £15 billion over the last ten years, and when you take into account all the equipment programmes such as the Typhoon, the A400M transport aircraft and the rest, where is the money going to come from?”

In a move to ease the financial pressure, the Treasury has agreed to allow the MoD more flexibility in its budget that will free millions of pounds from other spending years to devote to the carriers. In return, the MoD has had to promise to hold a review of all other equipment projects. “Long-term planning has been abandoned in a panic attempt to plug the gaping hole in the defence budget,” Liam Fox, the Shadow Defence Secretary, said.

It means that the MoD is expected to announce the very long-awaited contract to build the two carriers for £3.8 billion, putting the Royal Navy in the big league of blue-water navies, raising its carrier profile from the relatively minor Invincible class to the sort of tonnage displayed by the Americans. The nuclear-powered USS Nimitz-class ships, the biggest in the world, are each 91,500 tonnes fully loaded.

“With the Invincible-class carriers, Britain has been punching below its weight, but with the larger carriers we will be punching in accordance with the weight we should be demonstrating as one of the five permanent members of the Security Council,” a navy source said.

The Navy puts up a stout defence of the programme and insists that size is not what matters, but capability. “Whether the carrier is 40,000 tonnes or 65,000 tonnes doesn’t make much difference in terms of cost because steel is relatively cheap. What’s important is that we have a platform that can take 36 aircraft,” it said.

The MoD is now beset with budget problems. It has become overwhelmed with equipment programmes it cannot afford and something has to give.

The Navy’s response is the one that the Government has been emphasising ever since the big carrier option was taken up ten years ago — that Britain will not be able to rely on a friendly host nation from which to launch land-based fighters and bombers in some future regional conflict. A carrier with an extended flight deck will provide the flexibility and capability required to operate from international waters with relative impunity. They will also be the largest ships yet sailed by the Royal Navy.

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