Brown promises to scale back Trident

World powers move to endorse Obama’s nuclear-free vision as PM offers to cut the number of Trident nuclear submarines

The leaders of the world's greatest powers, including the Prime Minister Gordon Brown, are set tomorrow to endorse President Barack Obama's ambitious goal of ridding the world of nuclear weapons. According to the final draft of a resolution to be put to a rare summit of the five permanent members of the UN Security Council, the leaders will resolve "to seek a safer world for all and to create the conditions for a world without nuclear weapons".

In an overt show of support, Mr Brown will unveil plans to cut the number of Trident nuclear submarines from four to three. The Prime Minister will tell the special session of the Security Council, which also includes the leaders of France, Russia and China, that he is "prepared to consider" the move. Mr Obama convened the extraordinary summit-level session of the Council to give impetus to his mission to denuclearise the planet as the crisis surrounding Iran and North Korea and their rush to acquire atomic weapons appears to be deepening. The text of the resolution, seen by The Independent, is likely to be hailed as a breakthrough by campaigning groups. President Obama first spelled out his dream of counting down to a nuclear-free world in a speech in Prague at the end of March. For his proposal now to be adopted formally by the UN Security Council amounts to a significant endorsement by the world's leading powers. It is given added weight by the support of the non permanent members of the council, including Vietnam, Japan, Mexico Turkey and Libya, which was until recently an aspiring nuclear state.

"The only way to eliminate the nuclear threat is to eliminate all nuclear weapons," former US Ambassador Thomas Pickering of the Global Zero campaigning group said last night. "The Security Council's endorsement of this goal would be another great step toward an international consensus on working together to achieve this important objective."

In Britain, Trident is due to be upgraded in a £25bn programme approved by the Tony Blair's government with the backing of then-Chancellor Brown. The UK's arsenal of warheads has already been cut from 200 to 160 and scrapping one boat would not mean it could be cut further as the Government's policy is to retain the minimum number needed for an effective deterrent. A final decision will rest with the full Cabinet, but Mr Brown's plan looks certain to be approved. His aides insisted the aim to ensure Britain played its full part in global disarmament talks rather than simply a way to save up to £5bn. However, the planned scaling down of Trident is the most graphic sign of the public spending cuts to which Mr Brown committed himself last week to balance the nation's books.

By restating his commitment to retaining Britain's independent nuclear deterrent, he will disappoint critics who describe Trident as a Cold War relic. Aides described that as "non-negotiable" because of the threat that a rogue state might acquire nuclear weapons.

Tomorrow will see only the fifth Security Council meeting at heads-of-government level – and the first with a US president in the chair. The draft resolution congratulates the commitment made by the US and Russia to cut their arsenals further as they renegotiate the 1991 START treaty that is soon to expire. As in Prague, the resolution will also reaffirm the importance of the Nuclear Non-Proliferation Treaty (NPT), which comes up for review in May and cautions against nuclear materials falling into the hands of terrorists. The leaders, it says, are "gravely concerned about the threat of nuclear terrorism".

Also in the text are clear warnings to Iran and North Korea as they continue to defy calls to relinquish their nuclear programmes. It expresses "particular concern at the current major challenges to the non-proliferation regime" and demands the "parties concerned comply fully with their obligations". Efforts by Britain and the UN to name Iran and North Korea in the text were thwarted by Russia and China. Indeed, the worsening stand-off with Iran threatens to eclipse the adoption of the resolution tomorrow.

Ominously, Iran's energy agency chief, Ali Akbar Salehi, announced yesterday that his scientists had perfected a new generation of centrifuges for enriching uranium and is testing them. "Chains of 10 centrifuges are now under test and will be gradually increased," he said. Iran's President, Mahmoud Ahmadinejad, who will also address the UN general Assembly today has repeated in recent days that he will not cede the "sovereign right" to develop nuclear technology, which he says is strictly civilian in nature.

The firebrand leader's hand may be strengthened if the coalition of Western countries pushing for new sanctions against Iran weakened. Last night, the consensus appeared to be in jeopardy after France's Foreign Minister, Bernard Kouchner, indicated that he would not support a US proposal to impede Iranian imports of refined fuel. "It is not my favourite at all," he said of such a step, saying it would "be a step to a full-blown blockade".

The five permanent members of the Security Council, joined by Germany, are set to discuss their position ahead of new talks with Iran that have been set for 1 October in Geneva. The US has quietly promoted the idea of a petrol embargo against Iran as the punishment that would have the biggest chance of hurting the regime.

Diplomats had thought their biggest challenge would be to persuade Russia and China of the wisdom of such a harsh move. Wobbling by France will take them by surprise and infuriate Israel, which has been threatening a military strike against Iran's nuclear power sites.

Factfile: Trident

Primary Function: Strategic Nuclear Deterrence

Entered service in 1994

Length: 13m

Weight: 58,500kg

Diameter: 1.9m

Range: More than 4,600 miles

Power plant: Three stage solid propellant rocket

Quantity: Fewer than 160 operationally available nuclear warheads, according to the National Audit Office

The UK deploys 16 missiles on each of its four Vanguard-class submarines

Destructive power: estimated as the equivalent of eight Hiroshimas.

Top speed: 6,096 metres per second

Cost: £16.8m per missile