Tony Blair faced mounting pressure to back down on plans to create "independent" state schools last night after John Prescott openly criticised them as a return to the 11-plus.
The Deputy Prime Minister said he feared poorer children could get left behind in a "first class/second class" system and even doubted the need for serious reform.
His decision to go public with his concerns has been seized on by critics of the proposals as proof of the depth of dissent with Labour ranks.
His words were warmly welcomed by MPs behind a set of alternative proposals launched last week and supported by more than 50 of Mr Blair's backbenchers.
With the number ready to vote against the changes said to be in three figures, Mr Blair faces a highly-damaging Commons defeat.
Unless he makes changes - and so far he has expressed his determination to press ahead with the reforms - he may have to rely on Tory votes to get his policy through.
The Conservatives urged him to ignore Mr Prescott's "class war" battle and stand firm if he wanted to secure changes he has called "pivotal".
In an interview with The Sunday Telegraph, Mr Prescott, who is known to have battled against the plans behind the scenes, said: "I'm not totally convinced major reform is necessary.
"Since I was an 11-plus failure, since I do believe that produced a first-class/second-class education system, I fear this is a framework that may do the same. I'm somewhat critical of it."
There was "a great danger" the new city academies could become grammar schools as they were dominated by better off families.
"My argument is that middle-class parents are concerned, and rightly so, about the quality of education for their children, which sadly is not the same for working-class parents," he said.
"If you set up a school and it becomes a good school, the great danger is that's the place they want to go to."
Mr Prescott failed his 11-plus and left secondary modern school at 15 but he later returned to education at Ruskin College and went on to acquire a degree at Hull University.
Mr Blair told MPs on Wednesday: "We will stick with the changes in the White Paper because they are the right changes to make."
But Downing Street has insisted the Prime Minister will listen to critics.
Europe minister Douglas Alexander said it was right to go ahead with reforms but they would be "informed by the discussions that we have with parliamentary colleagues".
Proposals would be better as a result of critical contributions from Mr Prescott and former education secretary Baroness Morris - one of those behind the alternative plan, he added.
"If you look at the past achievements of this Labour government over eight years, the pupils who benefited most from the reforms we have implemented have actually been the kids in the poorest communities and from families without educational achievements to their name in the past."
The Department for Education has said Education Secretary Ruth Kelly was meeting with MPs to discuss the reforms which she was determined would benefit more deprived areas first.
Labour MP Martin Salter, who quit as a parliamentary aide to education minister Jacqui Smith and one of the MPs behind the alternative proposals, said Mr Prescott's intervention was "most helpful".
He said he wanted a "healthy, vibrant debate" to find agreement and warned that changes would have to be made.
"I sincerely hope that Tony Blair will take note of the very, very serious paper that myself and colleagues have put together, the comments of Labour Party members, of headteachers, of school governors and, of course, of his own deputy.
"I can't believe Tony Blair won't listen to people who are on his side over education," he said.
Shadow education secretary David Willetts said Mr Prescott was fighting old "class war" battles and ignoring the plight of young people already denied a good education because of selection by house price.
"What we are seeing with John Prescott is a guy who has this deep resentment about not having passed the 11-plus and still, 50 years, 60 years on, he is fighting these battles.
"The real scandal that John Prescott should be angry about is not some strange idea we are going to return to the 11-plus, the real scandal is that there are still, in tough areas, in deprived areas, children whose parents can't afford the high house prices near to the good schools who are not getting a good enough education.
"Tony Blair and John Prescott between them are having an argument about an old issue. They are not talking about what parents care about: school standards."
He said the idea the Tories wanted a return to full-blooded academic selection like the 11-plus was a "non issue".
Teachers' union leaders backed Mr Prescott's concerns describing the plans as "absolute chaos" and a "return to the bad old days".