Tony Blair's position on Iraq suffered a setback on Wednesday as his government's motion sparked the biggest rebellion of his premiership.
An estimated 122 Labour backbenchers lined up to vote against the government.
A total of 199 MPs from all parties backed an amendment to the motion in which they said the case for war had not been proven.
But the government defeated the amendment thanks to the support of the Tories and the size of its own majority.
Labour chairman John Reid played down the vote.
"It is important to recognise what they were saying which was 'not yet'," he said.
"Well no-one was asking them to vote tonight to go to war and what's more three quarters of the people in the Parliamentary Labour Party, in the country actually take a different view."
But Labour rebel Graham Allen said the vote was a heartfelt plea from parliament for Mr Blair to "get off the treadmill to war".
Liberal Democrat leader Charles Kennedy stressed the significance of the vote saying: "At this crucial stage it sends a potent signal to the governments of both
Britain and the United States."
A further vote on the government's motion - effectively giving Saddam Hussein a final warning - was passed by 434 votes to 124 - another significant rebellion.
The division lists showed that 59 of those voting against the motion were Labour MPs.
The votes followed several hours of Commons debate on the crisis on how to deal with Saddam Hussein.
Mr Blair had tried to allay concerns by stressing he was working "flat out" for a second UN resolution and insisting MPs were not being asked to "vote for war".
'Not a war vote'
The Iraq debate was opened by Foreign Secretary Jack who said the government motion was "not an endorsement of military action by UK forces".
"No decision to deploy British forces has yet been taken," said Mr Straw.
The foreign secretary delivered a stern warning: "We are close to the crunch point.
"Saddam must either embark immediately on voluntary and full disarmament, or the security council has to face up to its responsibility to see that he is disarmed by force."
Backing the government's strategy, Conservative shadow foreign secretary Michael Ancram said Iraq would take "ambiguity as a sign of weakness".
Mr Kennedy argued that unless the weapons inspectors said their work had failed "it would be quite wrong to participate in pre-emptive action".
Labour backbench discontent was voiced by former Labour cabinet minister Chris Smith.
The ex-culture secretary said the government's motion would signal that MPs endorsed a timetable that would lead inexorably to war within three weeks.
Mr Smith told the Commons: "We must say here today in this chamber that now is not the time, that the case has yet to be fully made, and that war and all its consequences cannot be the present answer."
NEXT STEPS IN IRAQ CRISIS
24 Feb onwards: US-UK new resolution due
28 Feb or soon after: Blix written report to Security Council
1 Mar: Missile destruction must start
Around 7 Mar: Inspectors oral report to Security Council
10 Mar: US-UK will force UN vote on resolution
Earlier, Mr Blair tried to reassure MPs at prime minister's questions, when he insisted Parliament would be able vote "many times" on the crisis if there was military action.
Such votes would come before war unless that would jeopardise the safety of UK troops, he said.
'UN authority risk'
The US and UK this week unveiled their new draft UN resolution, but say it will not be put to a vote if Iraq cooperates "immediately and unconditionally" with UN resolutions.
The prime minister said the authority of the UN would be undermined if it failed to follow through its tough words.
With a second resolution seen as crucial to winning over public opinion, Mr Blair said he believed such a move would be agreed.