Today the Queen commissioned her namesake, the aircraft carrier HMS Queen Elizabeth, the largest ship ever built for the Royal Navy. She will be followed soon by Prince Charles, who can be expected to commission the navy’s second large carrier, HMS Prince of Wales.

The carriers were described to me by a former chief of defence staff as 'vulnerable metal cans'

The navy top brass are very proud, of course. The first sea lord, Admiral Sir Philip Jones, recently described the aircraft carriers and Britain’s fleet of nuclear missile submarines as “strategic instruments, indicative of an ocean-going navy and a global maritime power”.

In reality, the carriers and the new fleet of Dreadnought submarines are irrelevant to Britain’s security needs. They are of no use against terrorism, described by successive prime ministers as the most serious threat facing the country, or against cyberwarfare, which also preoccupies security chiefs. The carriers, like the nuclear missile submarines, are status symbols.

The carriers were described to me by a former chief of defence staff as “vulnerable metal cans”. The navy says that in a “high-threat environment” they will be protected by two destroyers, two anti-submarine frigates, a submarine, a tanker and a supply ship. That is a huge commitment for a navy that has just 19 destroyers and frigates and six available subs. And a growing body of military commentators say the carriers will be vulnerable to long-range, high-speed missiles being developed by Russia and China, cited by the Ministry of Defence as the two powers likely to threaten the UK in the future, as well as to attacks by underwater drones, a fast-developing technology.

The carriers and the fleet of new nuclear missile submarines are eating up a huge slice of the defence budget. Already under pressure, it is likely to face brutal cuts in the “capability review” due to be presented to the cabinet by Mark Sedwill, the government’s national security adviser, in the new year. Under threat are the Royal Marines and their two amphibious assault ships – HMS Albion and HMS Bulwark – precisely the sort of “agile” capability defence chiefs have been arguing Britain so desperately needs.

Facebook Twitter Pinterest The new aircraft carrier HMS Queen Elizabeth pictured today, as it was welcomed into the fleet by the Queen. Photograph: Si Ethell/EPA

The navy is already faced with the humiliating prospect of having two large aircraft carriers with very few aircraft flying from them, ever. They are designed to carry 36 joint strike aircraft – F-35s, known as the Lightning II – but are unlikely to carry more than 12. The cost of the US stealth aircraft has soared mainly because of serious technical problems. The cost of 48 the MoD has contracted to buy for both the RAF and the navy will amount to £9.1bn by 2025, rising to an estimated £13bn by 2048. The MoD will not give an estimate of the cost of the rest of the 138 planes it wants to buy, but it is very unlikely to be able to afford them.

Asked about the estimated cost of the planned full package, Stephen Lovegrove, the ministry’s top official, told the Commons defence committee last month that “it would be imprudent to put a number in the public domain which would inevitably be wrong”. It was a wise response. The MoD has a notoriously bad record in forecasting the cost of its ambitious weapons projects. The cost of the two new carriers was originally estimated at £3.9bn. It has turned out to be more than £6bn.

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The rising costs of the Dreadnought project, meanwhile, are forcing the MoD to find extra cuts of £300m in this year’s defence budget. Independent financial watchdogs have expressed doubts about whether the government can keep to the £41bn estimated cost of replacing the Trident fleet. Defence ministers have said they “do not recognise” the £200bn-plus cost of a new nuclear missile programme throughout its lifetime estimated by opponents of the project – but the MoD declines to offer its own estimate. That programme, and the plan to buy fighters for the RAF and navy, are Britain’s two most expensive projects. Yet we do not know how much either will cost.

The “capability review” now being carried out will inevitably lead to more cuts, at a time when the navy is short of sailors, the RAF short of pilots, the army the smallest since the time of Cromwell – and a huge slice of the defence budget is spent renewing a nuclear “deterrent” whose purpose is not be used, and on aircraft carriers with empty flight decks.

Such thoughts, however, were brushed aside when the white ensign was hoisted on HMS Queen Elizabeth today.

• Richard Norton-Taylor writes for the Guardian on defence and security