The Labour left scented blood last night as critics turned on Tony Blair following his humiliation in the Education Bill vote.
One said he had "effectively resigned" as Labour leader by voting with Tories in the face of opposition from his own MPs.
However, Mr Blair's supporters within the Parliamentary Party were outspoken in their support.
Birmingham MP Siôn Simon (Lab Erdington), who backed the Government, condemned the rebels.
He asked: "Is a quarter of working class kids getting a half decent basic education, is that good enough under a Labour Government? Should we just leave it where it is?"
Gisela Stuart (Lab Edgbaston) said the Government should never have made concessions to rebel MPs.
Ministers had added measures to the Bill banning schools from selecting pupils by ability and giving local councils a bigger role in education than originally planned, in an attempt to answer the criticisms of dissidents on the Labour benches.
Ms Stuart said: "The concessions are the only thing in the Bill I don't agree with."
But prominent rebel Bob Marshall-Andrews, MP for Medway, said the vote was "undoubtedly a defeat for the Prime Minister and the Government".
John McDonnell, the chairman of the left-wing Labour Representation Committee, said: "By relying upon Tory support in the face of his own backbenchers, Tony Blair has lost not just his moral authority but the confidence of the Parliamentary Labour Party.
"Tonight the Prime Minister has walked out on the Party and effectively resigned as leader."
Shadow education secretary David Willetts said the result of the vote showed Mr Blair was unable even to deliver "timid" reforms of public services.
Education Secretary Ruth Kelly played down the significance of the backbench revolt, insisting her legislation had "overwhelming support".
She said: "Three out of four Labour MPs supported this Bill. Two-thirds of the House of Commons voted for this Bill."
In a rousing speech immediately before the vote, Schools Minister Jacqui Smith, MP for Redditch, attempted to fire up Labour back-benchers by insisting the Bill was in keeping with the party's traditions.
It built on the work of former Education Secretaries including Estelle Morris, Charles Clarke and David Blunkett, she said.
"It is a vote for the aspirations of children and parents through the country. It is a vote for Labour values."
Bookmakers William Hill cut the odds against Mr Blair standing down as Prime Minister during 2006 from 2-1 to 6-4 in the wake of the vote.