Britain is prepared to scale back its nuclear capability as part of global disarmament efforts, Prime Minister Gordon Brown said Wednesday ahead of an address to the UN General Assembly.

He told BBC radio that Britain was prepared to reduce the number of submarines that can launch nuclear missiles, but said there were no plans to cut the number of warheads.

"Just as America and Russia are making those reductions, we are prepared to consider that, but only as part of an agreement," Brown said.

"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."

Brown said there should be tougher sanctions to stop states such as Iran and North Korea developing nuclear weapons, and they should also be offered help with civilian nuclear energy in return for dropping their weapons programmes.

"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," he said.

The prime minister was to outline plans to cut the number of submarines from four to three at a special session of the UN Security Council on Thursday, his office confirmed.

The meeting was to be devoted to the issue of nuclear non-proliferation, and presided over by US President Barack Obama.

But Brown added: "We are making no proposals at the moment about warheads."

The United States will also hold talks with its five counterparts -- Britain, France, Germany, China and Russia -- that have for years been negotiating with Iran on its nuclear programme.

"If we are serious about the ambition of a nuclear-free world we will need statesmanship, not brinkmanship," Brown was to tell the UN General Assembly in his speech, according to comments released by his office.

However, he was also to reaffirm Britain's commitment to maintaining an independent nuclear deterrent.

Britain's move comes ahead of next year's review of the Nuclear Non-Proliferation Treaty, which aims to limit the spread of nuclear weapons around the world.

It also follows a decision by Washington and Moscow to negotiate a successor to the landmark 1991 Strategic Arms Reduction Treaty (START), which bound both sides to deep cuts in their nuclear arsenals.

Brown's government has already outlined its willingness to cut nuclear arms before talks in 2010, to persuade Iran and North Korea to give up their nuclear ambitions.

Britain has the fifth largest nuclear arsenal in the world, according to experts.

The move on submarines could cut billions of pounds (dollars) from the defence budget over the next decade, but Foreign Secretary David Miliband rejected suggestions the move was financially motivated.

"Defence, and nuclear defence especially, is long-term questions not short-term financial issues," he told the BBC earlier.

Britain is preparing to renew its Trident nuclear weapons system at a cost of some 20 billion pounds (22 billion euros, 32 billion dollars) by the 2020s.

The Campaign for Nuclear Disarmament welcomed the plans to cut the number of submarines as a "serious and positive first step" towards the elimination of all nuclear weapons.

Liam Fox, defence spokesman for the main opposition Conservatives, added: "If we can maintain our independent nuclear deterrent and make a contribution to disarmament, then we should look at ways of doing so."