John Prescott has dealt a serious blow to Tony Blair's plans to give state schools in England more say over admissions, staffing and finances.
Mr Prescott failed his 11-plus and left secondary modern school at 15
The deputy prime minister told the Sunday Telegraph he feared pupils from poorer backgrounds could lose out under a two-tier, class-ridden system.
Mr Prescott, who failed his 11-plus but earned a degree, said: "I'm not totally convinced major reform is necessary."
Mr Blair has said he remains keen to introduce "independent" trust schools.
But BBC political correspondent Mark Sanders says such comments from the prime minister's deputy are embarrassing as Mr Blair tries to ward off a backbench rebellion on his school reforms.
A group of 58 Labour backbenchers this week published alternative proposals - entitled Shaping the Education Bill - Reaching for Consensus - that would give local authorities extra powers to coordinate admissions.
While Labour MP John McDonnell, who chairs the socialist Campaign group, said Mr Prescott had expressed deep concerns that were widespread across the party.
In the newspaper interview, Mr Prescott defended comprehensive schools and warned against any move that could pave the way for their abolition.
SCHOOLS WHITE PAPER
Allowing schools to become independent trusts, with more say over admissions, staffing and finances
Local authorities become "champions" for good schools, rather than education providers
Parents encouraged to set up their own schools
A "schools commissioner" and network of advisers to help parents
More help with transport costs for poorer families
"Don't write them off," he told the newspaper.
Mr Prescott failed his 11-plus and left secondary modern school at the age of 15, but he later studied at Ruskin College in Oxford, and graduated from Hull University.
Commenting on the government's White Paper on school reform, he said: "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."
BBC News website education editor Gary Eason said Mr Prescott's comments were all the more damaging because as well as criticising the latest proposals for change, he also reportedly cast coubt on the existing government commitment to have 200 city academies by 2010.
Asked about the new academies - state-run independent schools - Mr Prescott told the paper he thought there was a "great danger" that they could become grammar schools by another name.
Campaign chairman Mr McDonnell said the whole issue was one of "deep principle" and warned the prime minister he was "flying in the face of the whole of the Labour Party".
"I'm hoping the prime minister will listen now. If he doesn't, I think he faces defeat," he said.
On Wednesday, Mr Blair told MPs the changes in the White Paper would remain, as they were "the right changes to make".
The public school-educated prime minister has said the "pivotal" and "irreversible" reforms will create "a system of independent, self-governing state schools with fair funding and fair admissions... driven by the needs" of pupils and parents.
Councils get power to "refuse or restrain" setting up or expansion of schools where it might damage other schools
Local authorities to "coordinate" admissions for all schools
Idea of trust schools to be tested more thoroughly before passing into law
No need to establish commissioner, which duplicates role of local authorities
New Tory leader David Cameron has sought to exploit the Labour divisions by offering Tory support for the plans.
Speaking on BBC1's Sunday AM programme, shadow foreign secretary William Hague said the prime minister should take up Mr Cameron's offer.
"He (Tony Blair) knows that the only way to get results in say education, is to give greater freedom to schools at the local level. He knows that is the right thing to do.
"The question is, is he going to carry on and do the right thing, even if it means relying on the support of the Conservative Party, or is he going to give in to these old prejudices of John Prescott and others?"
Liberal Democrat education spokesman Ed Davey said: "The fact that John Prescott is willing to go public is mark of how unfair and damaging these reforms are, and exposes the serious splits in the Cabinet
"The government's proposals would undermine parental choice and would damage what parents really want - the right for their children to go to a quality local school.
Under the plans, local education authorities would have a more strategic role, monitoring standards and commissioning services rather than running schools.
Ministers insist the plans do not mean a return to selection by ability and that they will improve standards.