The World Court in The Hague yesterday set important limitations on the use or threat of use of such weapons. But the Court, which is the world's supreme judicial body, stopped short of an outright ban on the possession, use or threat of use of nuclear devices.
Last night, anti-nuclear pressure groups, including CND, were claiming victory in the close-run decision. But diplomatic sources took comfort in the Court's refusal to rule that the threat or use of nuclear weapons was illegal in all circumstances.
The Court ruled: "The threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict and, in particular, the principles and rules of humanitarian law."
However, it added: "The Court cannot conclude definitively whether the threat or use of nuclear weapons would be lawful or unlawful in an extreme circumstance of self-defence, in which the very survival of a state would be at stake."
Nevertheless, its unexpectedly robust decision delighted anti-nuclear campaigners, who had feared the Court would duck the issue.
The landmark decision has been keenly awaited by anti- nuclear groups and the five official nuclear powers since a large majority of the UN General Assembly asked for it in December 1994.
The case was also crucial to the authority of the World Court. The vote was balanced on a knife edge - seven judges voted for the decision, seven against, with a casting vote made by the Algerian President, Mohammed Bedjaoui.
It was the first time the International Court of Justice had been asked to rule on the legality of any weapon, but its decision had awesome implications.
If it had refused to make a ruling, it consigned itself to irrelevance and humiliation.
A decision that the threat or use of nuclear weapons was illegal would have made the policy of nuclear deterrence - on which the five official nuclear powers have depended for their security - illegal.
That did not happen yesterday, but threats by Britain and other nuclear- armed countries to use nuclear weapons in response to limited strikes or against third world states were ruled illegal, unless their very survival was at stake.
It has long been understood that the five official nuclear powers - the US, Russia, Britain France and China - would only use nuclear weapons as a weapon of last resort.
However, the rise of nuclear "threshold" powers - Iraq, India, Pakistan and Israel, which do have nuclear weapons but are not official members of the nuclear club - has given rise to doctrines of "sub-strategic" nuclear deterrence, in which small, highly-accurate nuclear weapons would be delivered against a renegade state or Third World country, possibly as a "warning shot".
Britain cancelled the RAF's tactical nuclear missile, which was partly intended for this role, but announced it would, if necessary, arm Trident missiles with single warheads to do the same job.
The World Court ruling effectively says that any form of "sub-strategic" nuclear deterrence is, by definition, illegal.
The Court has no mechanisms to enforce its judgment, but it is in tune with new military thinking.
The verdict will reinforce the pressure for less reliance on nuclear weapons which has been gathering momentum as more powerful and accurate conventional weapons become available, such as those used in the Gulf war against Iraq.
The US Navy recently published a paper stressing the need for massive and highly accurate conventional strikes to bridge the gap between operations by conventional armed forces and nuclear strikes.
The Royal Navy is to get US Tomahawk cruise missiles with high-explosive warheads to do a similar job, and the RAF ishoping to learn shortly which missiles suitable for hitting strategic targets from a safe distance it is to get. That decision could be made this week.
Though the verdict is in line with prevailing military trends, none of the established nuclear powers is yet ready to eschew nuclear weapons as its last resort. Nuclear deterrence is the policy of the present Government in Britain, and, Tony Blair reaffirmed two weeks ago, of a future Labour Government too.Reuse content