Britain's offer to scale back its nuclear deterrent does not include proposals to cut back on warheads, Prime Minister Gordon Brown said today.
Mr Brown will today tell the United Nations that he is prepared to consider cutting the UK's fleet of Trident missile-carrying submarines from four to three as part of a multilateral "global bargain" to reduce the risk of nuclear weapons.
But he made clear that he was making no proposals on further reductions in the UK's stockpile of Trident warheads, which has already been cut from 200 to 160 under Labour.
And he dismissed suggestions that his initiative was motivated by the need to save money and reduce the Government's deficit, saying: "Obviously there are cost implications in every decision, but that is not what is uppermost in our mind."
Mr Brown's offer comes as US President Barack Obama is pressing ahead with ambitious plans for international talks in Washington next March to halt the spread of nuclear weapons and reduce existing stockpiles.
In his address today to the UN General Assembly in New York, Mr Brown will call for all nations to come together to achieve the long-term ambition of a nuclear-free world.
"If we are serious about the ambition of a nuclear-free world we will need statesmanship, not brinkmanship," he will say, according to an advance extract from his speech.
He will then spell out the details of his offer tomorrow at a special session of the UN Security Council, summoned by Mr Obama, on disarmament and non-proliferation.
It will cover the planned £20 billion Trident modernisation programme - with the four existing submarines being replaced by just three boats.
Speaking to BBC Radio 5 Live ahead of his speech, Mr Brown said: "We face a race for nuclear weapons if we do not act now.
"We face a far less safe world with a number of states considering taking on nuclear weapons and equally the danger that nuclear weapons could fall into the hands of terrorist groups.
"That's why I am proposing today a new global bargain on nuclear weapons between nuclear weapons states and non-nuclear weapons states."
Asked by interviewer Simon Mayo if he was offering to scrap more of Britain's warheads, Mr Brown replied: "We are looking at the moment at the number of submarines and will continue to look at that in detail. We are making no proposals at the moment about warheads."
Mr Brown told Radio 5 Live that his "global bargain" involved three strands.
"One is that we have got to stop nuclear weapons getting into the hands of states who don't have them and therefore we have got to have a tougher regime of sanctions against countries like Iran and North Korea and we must put the onus upon these countries to prove that they are not developing nuclear weapons," he said.
"Second, we have got to say to states that are not going to develop nuclear weapons and are prepared to renounce them that we will help them get civil nuclear power.
"And third, in the spirit of the Non-Proliferation Treaty, we nuclear weapons powers have got to make some steps towards greater reductions in nuclear weapons.
"Just as America and Russia are making those reductions, we are prepared to consider that, but only as part of an agreement. That is why I have said we are prepared to consider - subject to all the conditions that I have laid down - reducing our number of submarines."
Officials travelling with the Prime Minister insisted the maintenance of the UK's independent deterrent remained "non-negotiable".
They cautioned also that reducing the number of submarines would not lead to a "proportionate" 25% cut in the cost of the programme as more would have to be spent on the remaining boats to maintain the overall deterrent capability.
Nevertheless they argued that the reduction would still send an "important political signal" in the run up to a review next May of the international Non-Proliferation Treaty.
Shadow defence secretary Liam Fox said Mr Brown's offer was nothing new.
"If we can maintain our independent nuclear deterrent and make a contribution to disarmament, then we should look at ways of doing so," said Dr Fox.
"But there is nothing new in what the Prime Minster is saying. In the Government's White Paper in December 2006, they said they would look at reducing our deterrent to three submarines.
"We have already said that we would consider reducing the number of submarines to three if we can maintain continuous at-sea patrols and the UK's credible nuclear deterrent with this number."
Liberal Democrats urged Mr Brown to go further and scrap plans to replace Trident entirely.
Foreign affairs spokesman Edward Davey said: "Once again, Gordon Brown is tinkering around the edges rather than taking the radical action we need to see.
"It is no longer possible to justify replacing one obsolete expensive Cold War nuclear system with another.
"Liberal Democrats have already ruled out like-for-like replacement of Trident. Gordon Brown should do the same, rather than persisting with empty gestures."