"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.
"This is exactly what the Non-Proliferation Treaty intended. In line with maintaining our nuclear deterrent I have asked our national security committee to report to me on the potential future reduction of our nuclear weapon submarines from four to three."
He added the UK would insist non nuclear states proved they are not developing weapons and could offer civil nuclear power to non nuclear states which were ready to renounce any plans for nuclear weapons.
But he said one of the greatest risks was that terrorists would acquire nuclear weapons.
Mr Brown also warned Iran and North Korea that the world would be even tougher on nuclear proliferation and was ready to consider further sanctions.
US President Barack Obama is chairing a Security Council meeting as part of the process of drawing up a replacement for the Non-Proliferation Treaty, designed to stop countries developing nuclear weapons.
David Miliband: "The long-term aim is a world free of nuclear weapons"
He has said he will try to negotiate with Moscow to reduce US and Russian nuclear warheads - which make up the vast majority of the world's total - from more than 2,000 each to 1,500.
The UK government says it has cut its stockpile of Trident warheads from 200 to 160 but many Labour MPs would like it to scrap the weapons altogether.
Earlier, Mr Brown told BBC Radio 5 live there were "no proposals at the moment about warheads".
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.
Professor Ron Smith, a defence economist at Birkbeck College, told the BBC that reducing the number of submarines would probably have little effect on Britain's nuclear capability because one was essentially "a spare".
The four Vanguard submarines which host Trident missiles can attack targets within a range of just over 4,600 miles (7,400km). The example above shows this range if the sub were located in the mid-Atlantic.
Britain keeps at least one submarine armed and at sea at all times. Normally, the remaining three - or two, should one be scrapped - will be undergoing maintenance, taking part in training exercises, or docked at port.
He added that losing one submarine would only save "a couple of billion" in about 2020 as there were a lot of fixed costs upfront and each boat cost less to build than the last one.
Foreign Secretary David Miliband said: "We reject unilateral nuclear disarmament for ourselves precisely because the world cannot end up in a situation where responsible powers get rid of their weapons, but the danger of nuclear proliferation by other powers remains."
But the former defence secretary John Hutton, in whose Barrow and Furness constituency the submarines are built, said it was "very, very important" that Britain could have "absolute confidence and the knowledge that we can maintain one of those submarines - at least one of those submarines - on continuous deterrent patrol".
The existing Trident submarines are housed at the Faslane naval base on the Clyde, but the Scottish National Party has firmly opposed their replacement.
Shadow defence secretary Liam Fox said reducing the number of submarines was not a new idea as it was an option set out in the government's 2006 White Paper.
He told the BBC: "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."
He said it was "reasonable and sensible" to look at doing that if the technology was available to make it possible.
The Liberal Democrats say they would not seek a "like-for-like replacement" for Trident - which is due to need replacing by 2024.
Leader Nick Clegg said it was good the prime minister had acknowledged the need for alternatives to a "like-for-like" replacement but he might have to go "a lot further".
However the party's foreign affairs spokesman Edward Davey told the BBC that the suggestion "looks like tinkering at the edges" of the nuclear weapons issue.
Bruce Kent, the vice-president of the Campaign for Nuclear Disarmament, said multilateral reductions "don't mean anything".
He added: "What does it matter if you have 100 nuclear weapons or 200 nuclear weapons - you could do horrendous damage, numbers don't really matter at all.
"British nuclear weapons have no function, apart from encouraging other people to get nuclear weapons."
All numbers are estimates because exact numbers are top secret.
Strategic nuclear warheads are designed to target cities, missile locations and military headquarters as part of a strategic plan.
Israeli authorities have never confirmed or denied the country has nuclear weapons.
The highly secretive state claims it has nuclear weapons, but there is no information in the public domain that proves this.
The International Atomic Energy Agency reported in 2003 there had been covert nuclear activity to make fissile material and continues to monitor Tehran's nuclear program.
US officials have claimed it is covertly seeking nuclear weapons.
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