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

Mr Brown, who presented the move as part of the "global bargain" for arms reduction, said: "If we are serious about the ambition of a nuclear-free world we will need statesmanship, not brinkmanship.

"All nuclear weapons states must reciprocally play their part in reducing nuclear weapons as part of an agreement by non-nuclear states to renounce them."

The UK Government has already cut its stockpile of Trident warheads from 200 to 160 but many Labour MPs would like it to scrap the weapons altogether.

The government estimates the cost of renewing Trident at about £20bn but Greenpeace says it could cost £34bn and, once lifetime running costs are included, would cost nearly £100bn in total.

While some in the military doubt the nuclear deterrent can be maintained with fewer than four submarines -- with a rota that keeps one boat permanently at sea -- there was a broad political welcome for Mr Brown’s announcement.

Shadow Defence Secretary Liam Fox backed the plan as "reasonable and sensible", pointing out that reducing the number of submarines was not a new idea as it was an option set out in the government’s 2006 White Paper.

"If we can maintain our nuclear deterrent and make a contribution to disarmament that’s all very well but the Prime Minister is not planning to reduce the number of warheads from 160, merely to have them in fewer submarines," said Mr Fox.

The Liberal Democrats said they would not seek a "like-for-like replacement" for Trident -- which is due to need replacing by 2024 -- and that the proposals were tinkering at the edge of disarmament, while SNP Westminster leader and defence spokesman Angus Robertson MP said Mr Brown had missed the opportunity to lead the world on non-proliferation and make real budget savings

"Instead of offering a vague pledge to make a token reduction Gordon Brown should make a firm commitment to eliminating nuclear weapons," said Mr Robertson. "The fact is that the Prime Minister has been totally inconsistent on nuclear arms limitation -- he is lecturing the world on non-proliferation while simultaneously planning to pay a £2bn down payment on Trident replacement."

CND welcomed the proposals as "a serious and positive first step" but several Labour MPs like Nigel Griffiths, the Edinburgh South MP who resigned as a junior minister over the Trident vote in parliament, called on the Prime Minister to go further.

"My view is clear, if you have former hawks like Henry Kissinger saying there is no rationale for funding nuclear weapons in the 21st century because the nature of the threat has changed, then it really is time for government to think again.

"That, combined with the funding commitment which could be much better spent in other areas, including defence, [means] it is time to call a halt to Trident."

The Church of Scotland and the Catholic Church in Scotland broadly welcomed the

move to reduce the deterrent.