John Prescott last night sought to inflict maximum damage on Tony Blair's key education reforms, claiming they risk creating a two-tier schools system.
The Deputy Prime Minister used an interview to go public over his opposition to Mr Blair's most cherished domestic policy. He said he was not convinced that reform was necessary and that there was a "great danger" it would lead to a return to grammar schools.
An 11-plus failure himself, Mr Prescott said measures to free schools from local council control could disadvantage children from working-class families.
His intervention, designed to speak directly to Labour rebels, means Mr Blair cannot now expect the support of his most senior ally to push an Education Bill through the Commons in the new year.
Mr Blair insisted last week that he would press ahead with his plans for a new generation of "independent" state schools. But Mr Prescott has now publicly echoed critics' warnings.
Although it was known that he was privately unhappy about the reforms, his public intervention spells serious political trouble for Mr Blair's authority.
"I'm not totally convinced major reform is necessary," he told Susan Crosland, widow of the former Labour education secretary Tony Crosland who interviewed him for The Sunday Telegraph. "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. That's why I expressed my view in Cabinet about it ... Given the White Paper I fear there may be movement to that."
There was "a great danger" the new city academies could become grammar schools by another name, he said, adding that comprehensives should not be written off.
"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.
"I have a different view about some of the education reforms and I have expressed it in some inner circles," he said, conceding that his fears were not "proven".
Mr Prescott left his secondary modern school at 15 but he later returned to education at Ruskin College, Oxford, and went on to graduate from Hull University.
Downing Street has insisted that Mr Blair would listen to the concerns of the backbenchers, who claim the rebellion could reach into three figures.
But in the Commons on Wednesday, he told MPs: "We will stick with the changes in the White Paper because they are the right changes to make."
The rebel group - including several ex-ministers such as a former secretary of state for education, Estelle Morris - fear pupils from poorer areas will lose out as popular schools expand and wealthier parents are able to set up their own schools, operating their own admissions policies. The rebels unveiled their alternative plans in last week's Independent on Sunday.