Robin Cook, a British cabinet minister who resigned earlier in the day, said late Monday that he could not support a war without UN backing.
Cook, former Leader of the House of Commons, told lawmakers that the reason he resigned from the government was that he could not back a march toward a war with Iraq that did not have international and domestic support.
In a statement to the MPs in the lower house of parliament, Cook also warned that international alliances of all kinds were under threat now that the diplomatic route had been abandoned.
Iraq's military strength was less than half what it had been atthe time of the last Gulf War, Cook said, adding that it was illogical to argue, therefore, that Iraq presented a threat and that threat justified war.
Iraq probably had no weapons of mass destruction in the "commonly understood" sense of being a credible threat that could be delivered on "a city target," he added.
Cook also said he would vote against the government's stance on Iraq on Tuesday.
British Foreign Secretary Jack Straw announced late Sunday that a parliamentary vote on military action against Iraq was expected to be held at 10 p.m. (22:00 GMT) Tuesday.
Cook handed in his resignation Monday afternoon, minutes before British Prime Minister Tony Blair's emergency cabinet meeting on Iraq.
His resignation came shortly after Britain's Ambassador to the United Nations Jeremy Greenstock announced that diplomatic routes to resolve the Iraq crisis had been closed.
Local analysts said Cook's resignation might inflame anti-war Labor lawmakers into a massive show of dissent during a parliamentary vote on war in Iraq set for Tuesday.