Brown offers to cut Trident nuclear submarines by a quarter

A Royal Navy Trident nuclear submarine. Photograph: Corbis

Gordon Brown will add momentum to moves towards nuclear disarmament tomorrow by announcing that he intends Britain to build only three, and not the planned four, replacement Trident nuclear submarines.

The move, which could cut billions from the defence budget over the next decade, was welcomed by anti-nuclear campaigners today, who said it was a "step in the right direction" but did not go far enough.

In a speech to the UN general assembly today, Brown will say it is time for "statesmanship, not brinkmanship" on nuclear disarmament if the ambition to create a nuclear-free world is genuine.

He will outline the details of Britain's unilateral gesture at a UN security council nuclear non-proliferation conference hosted by Barack Obama in New York tomorrow.

The UK has already reduced the overall explosive power of its nuclear arsenal by 75% since the cold war. Sources added that savings to the Ministry of Defence budget might be £3bn to £5bn, and even more in reduced running costs. But they stressed that the Trident move was not primarily designed to cut costs, and savings may be hard to predict.

The Campaign for Nuclear Disarmament (CND) welcomed the news but called for the government to abandon plans to renew Trident.

"By failing to disarm, we encourage proliferation and put ourselves – and the whole world – at greater risk," Kate Hudson, the CND chairman, said. "The government has understood this, and has taken a step in the right direction. But the disarmament process cannot stop here – the government must press ahead courageously and scrap Trident."

The announcement has been agreed with senior officials at the MoD, and the issue will be referred by Brown to the national security cabinet committee for endorsement by December.

The committee will also look at the current plan to retain 160 warheads. It is feasible to keep all the warheads with a reduced number of submarines. But some cabinet members will be disappointed Brown is not willing to abandon the British deterrent altogether, and will press for some movement on reducing warheads.

The move is understood not to be conditional on major new disarmament offers by other nuclear states. Final decisions on the Trident contract probably do not need to be made until 2012, with the fleet becoming operational in 2025. British officials travelling with the prime minister said the decision was not necessarily the last disarmament offer to be made by Brown ahead of the general election.

The reduction from four to three submarines would still allow the continuous operational use of the submarine fleet, and is not a precursor to abandonment of the independent deterrent.

Brown is making his move ahead of Labour conference next week and the international nuclear non-proliferation review conference next May. The cost of Trident has been set out at between £72bn and £92bn over the next 20 years, including maintenance.

Polls, including one published this week by New Labour website Left Foot Forward, show growing support for the total scrapping of the UK deterrent. Only 23% wanted to replace Trident with an equally powerful weapon given the budget pressures of the continuing Afghan deployment.

Last night Lib Dem leader Nick Clegg told Newsnight: "I really do welcome that finally the dam has burst on this. I have been saying for months it is just unrealistic for us to believe that we can foot the £100bn like-for-like replacement cost for Trident over the next 25 years. I think the strategic context in which that decision is taking place is very different."

Brown had announced a wider strategic defence review for after the election. But he has been impressed by the speed with which Obama has moved to grasp nuclear disarmament. Last week Obama announced he was not going ahead with a nuclear weapons shield in Poland; he has also ordered officials to look at cutting the stockpile from 2,100 warheads to a figure in the hundreds. Brown has been backing a new non-proliferation regime in which the onus is placed on non-nuclear weapon states to prove they are in compliance with the treaty. In return, the states would be given access to uranium for nuclear power. In an opinion piece in the New York Times today, he described the offer as a "grand bargain" and said a new treaty was needed urgently because the world was threatened "by a looming new arms race".

Critics of Brown will argue his move to reduce the deterrent is the minimum possible, given the international disarmament momentum. But the initiative has come earlier than some expected.

Professor Ron Smith, a defence economist at Birkbeck College, told BBC Radio 4's Today programme that as the fourth submarine was designed for "insurance" the move probably did not make any difference to international disarmament "except politically". Given that Trident is not due to be replaced until 2020 he added the decision "was not going to help the [budget] deficit in the short-term".

In his speech to the UN general assembly, Brown will argue that "the world is at a point of no return" and needs leaders to grasp the issues of the next six months with the same focus that marked the banking crisis. He will argue that the world needs his brand of progressive multilateralism, and will suggest that the "little Englandism" of David Cameron is inappropriate. Procuring three, rather than four Trident nuclear missile submarines, is the easiest and cheapest option open to the government, defence officials and independent analysts say.

It was signalled in the government's 2006 white paper on renewing the multi-billion Trident system and would save a very small amount of this,they say.

The white paper said that designing and building four new submarines would cost between £11bn and £14bn. This is set against the estimated £70bn or more defence officials say the proposed replacement of the existing Trident system, including new missiles and warheads, would cost over its 30-year lifetime.

Greenpeace said in a detailed study published last week that a new Trident system would cost £97bn when all running costs, including ships and satellites deployed to protect the submarines, were taken into account.

Brown's announcement reflects growing opposition among defence chiefs, particularly in the army and the RAF, to the government's decision to renew the Trident system at a time when the defence budget is under huge pressure because of the need to provide troops fighting the Taliban and other insurgent groups on the ground in Afghanistan.

Even those military chiefs in favour of maintaining some kind of nuclear deterrent question the government's insistence that the existing Trident system should be replaced by a new fleet of submarines carrying long-range ballistic missiles with nuclear warheads.

Brown's concession is unlikely to carry any conviction with states, such as Iran, which argue that the official "nuclear club" should do more to show they are willing to take significant steps to disarmament.

More significant as far as Britain is concerned would be to cut the number of nuclear missiles and warheads. Reducing a new Trident fleet from 3 to 4 submarines would save about £3bn.