Tony Blair last night faced the largest defiance of his authority since he became Prime Minister in 1997.

In a Commons debate on Iraq, 121 Labour MPs voted against military action to strip Saddam Hussein of his weapons of mass destruction.

Instead, they backed an amendment tabled by former cabinet minister Chris Smith, which said the case for war has not been made.

Among them were North-East MPs Ronnie Campbell (Blyth Valley), Jim Cousins (Newcastle upon Tyne Central) and Doug Henderson (Newcastle upon Tyne North).

Stockton North's Frank Cook also supported the amendment but was unable to vote last night.

He said: ''Any war in these circumstances would not be justified. I have no affection for Saddam Hussein, but we can't go ahead without the UN.''

The Labour MPs were backed by a handful of Tories, including former Chancellor Kenneth Clark and Douglas Hogg, who was agriculture minister, along with 50 or so Liberal Democrats.

The scale of the vote for the rebel amendment saying the case for military action was ''as yet unproven'' was significantly higher than had been predicted.

It leaves Mr Blair facing the prospect of a devastating split in the party if he does go to war - particularly if he does so without the backing of a second United Nations resolution.

The Government defeated the rebel amendment comfortably enough - by 393 to 199 - thanks to a combination of the ''payroll vote'' of ministers and their aides and the backing of the vast majority of Tory MPs.

A second vote, on the Government's policy of seeking to disarm Iraq through the UN, was passed by 434 to 124.

Labour Party chairman John Reid sought to play down the revolt, saying that the Government had not been asking for a mandate for war at this stage and that there was still time for Saddam Hussein to disarm peacefully.

''It is roughly what I would have expected from meeting people throughout the country and also meeting people inside the Labour Party,'' he told Channel 4 News.

''It is about a quarter of people who are not yet convinced, they say the case has not been proven, and about three-quarters who are prepared to countenance military action if it becomes essential.''

However, one leading rebel, Labour backbencher, Graham Allen, immediately urged the Prime Minister to heed the voice of his own MPs and pull back from military action.

''It is a heartfelt plea from Parliament to the Prime Minister to get off the treadmill to war,'' he said.

''People here are willing to support the Prime Minister but not George Bush.''

Charles Kennedy, the leader of the Liberal Democrats who joined the Labour rebels in voting against the Government, said MPs had sent a ''potent signal''.

''This is a very significant parliamentary occasion. Despite investing masses of political and parliamentary capital, the Government has still failed to persuade a third of the House of Commons,'' he said.

''At this crucial stage that sends a potent signal to the Government of both Britain and the United States.''

Last night's revolt was more than twice the previous biggest rebellion under the present Government, representing more than a quarter of all Labour MPs and a third of Labour backbenchers.