A MASS mutiny by more than 120 Labour MPs over war with Iraq left Tony Blair facing a perilous moment in his premiership last night.

The rebels were among 199 MPs — almost a third of the Commons — who voted against early military action to disarm President Saddam Hussein. It was the biggest revolt against any governing party in parliamentary history and it served notice on the Prime Minister that he will have to win a second UN resolution to avoid his future being called into question.

Amid dramatic scenes in the Commons, 121 Labour rebels joined 13 Conservatives and 52 Liberal Democrats to vote for an amendment declaring that the case for military action had not yet been made.

The rebel amendment was defeated by 393 votes to 199, a majority of 194. The government motion backing its UN efforts to disarm Saddam was passed by 434 votes to 124. On that the revolt fell to 59 MPs.

Despite last-minute arm-twisting by the whips and pledges by Jack Straw and Mr Blair that MPs would have another opportunity to debate war, the Labour revolt dwarfed the worst of the last Parliament — on lone parents and disability benefit.

Even more worrying for Mr Blair were the private warnings from dozens of Labour MPs who remained loyal yesterday that they would be unable to support him if he sought to go to war without UN authority. In those circumstances Mr Blair would probably need Tory MPs’ support to secure Parliament’s backing for military action.

Acknowledging that before the debate began, Mr Blair told MPs that he was working “flat out” to secure the passage of a UN resolution which concludes that Saddam had failed to take his “final opportunity”.

But Mr Blair now clearly faces a devastating split in his party if he goes to war without UN approval. The Times was told authoritatively last night that by lunchtime yesterday Labour whips had expected 145 MPs to rebel. They managed to dissuade about 20 of them and believe that the rebel tally would fall dramatically if a second UN resolution were passed.

Downing Street was taken aback by the scale of the revolt, but insiders said that it was unlikely to sway Mr Blair. “You have seen what he is like. He believes he is doing the right thing,” one said. John Reid, the Labour Chairman, said: “It is roughly what what I would have expected from meeting people throughout the country and inside the Labour Party.”

Charles Kennedy, the Liberal Democrat leader, said MPs had sent a potent signal. “This is a very significant parliamentary occasion,” he said. “Despite investing masses of political and parliamentary capital, the Government has still failed to persuade a third of the House of Commons.”

Mr Blair and President Bush have embarked on a frantic campaign to persuade waverers in the 15-member Security Council to support the resolution when it is put to the vote the week after next.

Mr Blair voiced his hope and expectation that the resolution would be passed and ministers privately believe that the Security Council will be persuaded and that France will refrain from exercising its veto. But ministers admit that if the count were taken now, Britain and the US could not be sure of winning the day.

Mr Blair also appeared to shift position on his past insistence that he would go to war without the UN only if there was an “unreasonable” veto in the face of clear evidence that Saddam had not fulfilled his obligations under previous UN resolutions. Yesterday he said that any veto would be unreasonable if Iraq were in material breach. Resolution 1441 had made it absolutely clear that Iraq had a final opportunity to disarm. If it did not comply, it was in breach.

In the Commons, the opposition to military action was spread across the parties with the former Conservative Chancellor Kenneth Clarke and the former Labour Cabinet ministers Chris Smith and Frank Dobson leading the way.

Mr Clarke called for more time to be given to diplomatic efforts to disarm Saddam. He was a strong supporter of the Atlantic alliance and not some “anti-American, left-wing, peacenik”, but it was time to put down a marker and say that the “other approaches — diplomatic, deterrent policy, the use of threat to get compliance, have not yet been exhausted”.

Leading the case for the Government, Mr Straw invoked the UN’s predecessor, the League of Nations, saying: “The League failed because it could not create actions from its words. It could not back diplomacy with the credible threat and, where necessary, the use of force.

“So small evils went unchecked, tyrants became emboldened, then greater evils were unleashed. At each stage good men and women said ‘not now — wait, the evil is not big enough to challenge’. Then, before their eyes, the evil became too big to challenge. We had slipped down a slope, never noticing how far we had gone until it was too late. We owe it to our history as well as to our future not to make the same mistake again.”

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