In a major setback to British Prime Minister Tony Blair, two of his ministers resigned on Tuesday in protest against the government's policy over Iraq.

Junior Health Minister Lord Hunt and Home Office Minister John Denham announced their resignations as Blair pleaded for his Labour Party's backing for joining the United States in military action against Iraq and said that the crisis will determine the shape of international politics for a generation.

"Back away from this confrontation now and future conflicts will be infinitely worse and more devastating in their effects," said Blair opening a critical debate in the House of Commons, which is effectively being asked to sanction Britain's participation in a war to disarm Saddam Hussein.

However, Blair got a boost when International Development Secretary Clare Short, who had earlier threatened to resign on the issue, changed her mind but said she was still "very critical" of the way the crisis had been handled.

Lord Hunt said he resigned because he did not support "the pre-emptive action that is going to be taken without broad international support or indeed the clear support of the British people".

Denham said he was quitting because he could not vote in support of the government.

"I met the Prime Minister this morning to explain my reasons," he said, adding he hoped to speak during a 10-hour debate on Iraq in the Commons.

Leader of the Commons Robin Cook had resigned on Monday. In a statement, Cook said he could not back a march towards a war that did not have international and domestic support.

"Neither the international community nor the British public are persuaded that there is an urgent and compelling reason for this action in Iraq," he said.