Tony Blair insisted during the election campaign that he had not yet made a final decision. But aides have taken more heed of a line in Labour’s manifesto stating “we are committed to retaining the independent nuclear deterrent”, as well as the Prime Minister’s subsequent remark that this is “the right thing for the country”.

The likelihood of a replacement being ordered within this Parliament was flagged up by the Defence White Paper back in December 2003. Officials have been operating for several months on the assumption that by the next election the Government will have approved a new generation of weapons to replace the existing submarines armed with Trident missiles.

Although the system will not become obselete for 20 years, work on its successor needs to start soon. It is understood that a substantial contingency fund has been built into the Ministry of Defence budget for this purpose. The Atomic Weapons Establishment in Aldermaston has been busy recruiting scientists to work on the project.

Britain builds its own warheads at Aldermaston, but the delivery system — like Trident — would almost certainly be purchased from the United States. The 1958 mutual defence agreement, which gives Britain unique access to American nuclear technology, was renewed last autumn to facilitate the replacement of Trident.Ministers recognise that the multibillion-pound bill will be controversial, not least because the most obvious case for maintaining a deterrent disappeared with the end of the Cold War.

Today, as the Nuclear Non-Proliferation Review Conference draws to a close, campaigners will present a petition to No 10 urging Mr Blair to scrap Britain’s nuclear arms.

Jeremy Corbyn, the left-wing Labour MP, claimed yesterday that the non-proliferation treaty required nuclear powers to take steps towards the abolition of existing capabilities. “To comply, the Government must stop any research and design work on a new generation of nuclear weapons at Aldermaston and make an un- equivocal statement that it will not replace Trident,” he said.

Such arguments are likely to get short shrift from Mr Blair and his likely successor, Gordon Brown — both of whom remember the unpopularity of Labour’s unilateral disarmament policy in the 1980s. In an interview this month, the Chancellor said: “The issue in the world is not whether the existing powers cease to be nuclear — I think the issue is whether we can prevent proliferation.”

The Government believes that countries such as North Korea or Iran developing nuclear capability represent a new threat that must be deterred, while a decision to leave the club of nuclear powers would diminish Britain’s international standing and influence.

The White Paper of 2003 indicated the need for a decision on Britain’s nuclear future in this Parliament. The Whitehall inquiry is thought to have focused on a submarine-based solution. Replacements for the existing Vanguard-class submarines could be built in British shipyards, helping to safeguard jobs. Officials are also keen to avoid the potential protests that might be focused on a land-based delivery system such as the Americans had at Greenham Common in the 1980s.

There has also been discussion over the merits of buying a more tactical nuclear weapon. One option would be to install nuclear-tipped cruise missiles in the Royal Navy’s Astute-class submarines, at present being built at Barrow-in- Furness. But this scheme would prompt concerns over whether a future government would be more tempted to fire such missiles than the intercontinental ballistic systems that guaranteed “mutually assured destruction” during the Cold War. The cruise missile would also be difficult to reconcile with a US-Russian deal to take such weapons out of service.

The least controversial solution would be to upgrade the four Vanguard-class submarines and the warhead in the Trident D5 ballistic missiles. This could allow Trident to remain in service for another four or five years after 2025.

Duncan Lennox, the editor of Jane’s Strategic Weapons Systems, said that there were no realistic alternatives to the submarine-launched nuclear deterrent. Neither air-launched nor ground-launched systems offered the same concept of the “hidden deterrent”. Mr Lennox said that were the Government to go for the cruise missile option, the nuclear warhead could not be tested other than by computer modelling, Britain being a signatory of the Comprehensive Test Ban Treaty.

A spokesman for the AWE said yesterday that its mission was to support the Trident nuclear deterrent and to retain the capability to develop a successor if the Government requires it.

The Vanguard submarines that maintain the nuclear patrol round the clock are equipped with what is judged to be the minimum number of missiles and warheads to retain a credible deterrent. Each boat has at least one missile with only a single warhead with a sub-strategic range, to deter a rogue missile state that might threaten Britain.

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