The International Development Secretary says Britain should not take part in any military action against Iraq which is not authorised by the UN.
Clare Short says the UK's "duty" is to act as a restraining influence on Washington.
She added that is was also to ensure that the crisis over Saddam Hussein's weapons of mass destruction was dealt with through the UN.
Ms Short said: "The role of the UK in this historic and dangerous time is to try to keep the US with the UN process, to back up the authority of the UN and not divert from the UN route."