Senior government minister Robin Cook quit Monday as a revolt inside the ruling Labor Party escalated over Prime Minister Tony Blair's backing for military force to disarm Iraq.

Cook, one of Labor's most prominent figures, said he could not back a war that did not have the backing of the United Nations and a majority of the British people. Opinion polls show a majority of Britons oppose a war without United Nations approval.

"I can't accept collective responsibility for the decision to commit Britain now to military action in Iraq without international agreement or domestic support," said Cook, who was the government chief in the House of Commons.

Cook, 57, was foreign secretary in Blair's first government in 1997, but was replaced by Jack Straw in 2001.

In a surprise development, international aid minister Clare Short did not resign despite widespread expectations she would go after criticizing Blair's Iraq policy as "reckless." Short was considering her future but remained in the Cabinet, aides said.

The House of Commons was to debate the government's policy Tuesday, with Blair expected to get a harsh grilling from party rebels.

Blair was confident of retaining the support of a majority of his lawmakers and he can depend on the votes of the opposition Conservatives. Still, the party rebellion is a major embarrassment for Blair, who is taking the biggest political gamble of his career in opting for war.

Many Labor lawmakers say there is no justification for war, and the party's left wing is particularly unhappy about Blair's support for President Bush. About one-third of the party's lawmakers voted against the government last month on the question of whether war was justified.

Cook later urged lawmakers to oppose the government's war plans, receiving a rare standing ovation in the House of Commons. He blasted the government's decision to abandon a diplomatic solution.

"The reality is that Britain is being asked to embark on a war without agreement in any of the international bodies of which we are a leading partner," he said.

While saying he would vote against the government on Iraq, Cook said he did not back any campaign to oust Blair, hailing him as an outstanding leader.

Blair will hope to limit the vote against him Tuesday by stressing he made every effort to find a diplomatic solution. Britain lambasted France on Monday, saying Paris made war all but inevitable with its threat to veto any U.N. measure to set an ultimatum for Iraq.

Britain announced Monday along with the United States and Spain that a proposed resolution seeking U.N. approval for possible military action was being withdrawn.

There have been indications that many in the Labor Party, alarmed by the internal attacks on Blair, are trying to reduce the damage.

Blair is counting on a quick and successful war to sway public opinion and justify his policy. A long and bloody conflict, however, would damage Blair, possibly making it impossible for him to stay on.

The government's chief legal officer, Attorney General Lord Goldsmith, said Monday that the use of force is justified under three U.N. Security Council resolutions.

"All of these resolutions were adopted under Chapter VII of the U.N. Charter, which allows the use of force for the express purpose of restoring international peace and security," Goldsmith said in a statement.