More than 170 countries have signed the nuclear Non-Proliferation Treaty (NPT), which came into force in 1970 and must be renewed. A clear majority vote by its signatories is all that is needed. But the treaty has attracted political controversy. Some countries have refused to sign it, leading critics to allege that it is ineffectual. Many developing nations complain that it discriminates against them and one, India, has refused to attend the NPT conference, opening today at the United Nations.
Pressure groups, including the Campaign for Nuclear Disarmament and Greenpeace, say the nuclear powers themselves are in breach of the treaty's Article 6, which calls for progress towards disarmament. Some strategic experts believe an opportunity has been lost to secure a tighter regime, to prevent rogue nations from getting the bomb.
At the heart of the matter is the treaty's proposition that only five states - Britain, France, the US, Russia and China - are recognised nuclear powers. Other signatory nations pledge not to acquire nuclear weapons, and in return, receive the right to peaceful nuclear technology.
All the nuclear powers except China are agreed on their strategy at the conference. They want the treaty renewed for an indefinite period. China is thought to favour a "smooth extension" of the treaty.
The system, British officials say, has worked well. "The need is to renew the treaty unconditionally and indefinitely by a compelling majority," one said. "Without it, there would exist no international legal framework for the control of nuclear arms." John Holum, director of the US State Department's Arms Control and Disarmament Agency, said the NPT is "the cornerstone" of efforts to curb the spread of nuclear weapons.
"Approximately 40 countries would have the technical and material capability to build nuclear weapons if they wanted to do that," he said. The treaty had prevented them doing so. Many non-aligned countries have indicated they would prefer to renew the treaty for a fixed period of five, 10, 15 or 25 years.
That would provide leverage over the nuclear powers to fulfil their part of the bargain: disarmament and technology transfer.
British and American diplomats believe they will win a majority for indefinite renewal. But a vote may not be taken until early next month. Renewal by only a small majority would not grant the NPT the moral force Britain feels it deserves.
NPT supporters point to its successes. The treaty provisions compelled Iraq to accept the dismantling of its clandestine nuclear-weapons programme. South Africa destroyed its warheads and became a signatory. Argentina gave up plans to go down the nuclear road. North Korea has been constrained, if not prevented, from fulfilling all its nuclear ambitions by the censure of the International Atomic Energy Agency (IAEA). Mr Holum pointed out that without the NPT, there would have been no way to pressure North Korea.
A similar argument could be advanced over Iran, the focus of a dispute between Russia, which wants to sell nuclear reactors to Tehran, and Washington, which wants to prevent the sale. Western intelligence agencies believe Iran is operating a covert weapons programme and some believe that allowing the transfer of peaceful technology could allow the IAEA to enforce its regime of inspection and verification more thoroughly. Short of military intervention, they argue, there is little else that can be done.
Greater problems arise over the three "threshold states" that have not signed the NPT. Israel has accumulated an arsenal, estimated at more than 200 warheads, making it the only nuclear power in the Middle East. Arab countries, led by Egypt and Syria, say they should not be forced to sign a treaty extension while their powerful neighbour has not signed at all. The issue has entangled the NPT in the disputes of the Middle East and could lead to votes against indefinite renewal.
India and Pakistan, the other two "threshold states," are thought to be capable of making nuclear weapons. Their regional rivalry has brought the security dimension of south Asia into the argument. Neither has signed the NPT. India detonated a "peaceful" device in 1974 and Pakistan maintains a clandestine weapons programme. India professes to be concerned about China.
The five nuclear powers have sought to soothe their critics, by extending a new set of security guarantees to signatory states. These undertake not to employ nuclear weapons against non-nuclear powers and promise unspecified action, through the UN Security Council, to help any signatory that is under nuclear threat. A non-nuclear Egypt, for example, would be protected against Israel.
But non-aligned diplomats doubt the value of these assurances, noting that any of the nuclear powers could use its veto in the Security Council to block collective action. The 111 members of the Non-aligned Movement are to discuss their position in Jakarta on 26 April.
Whatever the outcome of the NPT conference, it will generate momentum for further strategic arms reductions by the nuclear powers. The US and Russia are likely to discuss new cuts in their arsenals at the Moscow summit next month.
A new negotiating process could soon pose a dilemma for Britain, whose four-vessel Trident nuclear submarine programme is said by ministers to constitute a "minimum deterrent".
Britain and France until now have kept their much smaller nuclear forces out of the strategic-arms negotiations.
But if a renewed NPT results in the greater emphasis on the commitment to disarm contained in Article 6, neither country may expect to remain immune indefinitely.