A legal pressure group told the UN Security Council yesterday that a preventive war against Iraq would be "both illegal and unnecessary",threatening the very foundation of international law and the United Nations.

"An action of such overriding importance can destroy international law," said Judge Christopher Weeramantry, former vice-president of the International Court of Justice.

In a petition presented yesterday, the International Association of Lawyers against Nuclear Arms said there was "no precedent in international law for use of force as a preventive measure, when there has been no actual or imminent attack by the offending State". Use of force is only permitted in the case of attack, or imminent attack, or if the Security Council declares a threat to peace and non-military measures have been determined to be inadequate, the association says.

Peter Weiss, the body's vice-president, said current UN resolutions offered "no pre-existing authorisation" for an Iraq war, adding that there was a precedent for the matter to be taken to the General Assembly.

Their position is likely to be contested by lawyers in the US administration, which argues that the current resolutions, alongside what they say is clear non-compliance by Iraq, give them the backing they need.

But it is unclear who is the final arbiter. Some argue that the Security Council holds the ultimate authority, but Mr. Weeramantry says it is subject to the UN Charter and the International Court of Justice.