The Government's former top education official has questioned Tony Blair's flagship reforms, warning that they could prompt more children to drop out of school.
Sir Michael Bichard, who was permanent secretary at the then Department for Education and Employment, suggested that the plans flew in the face of 50 years of evidence.
He was speaking after the Government published its controversial Education and Inspections Bill, which sets out moves to give schools more independence.
Sir Michael stressed he was not attacking Bill directly, adding there were many positive aspects to the legislation.
Sir Michael led the inquiry into the vetting of school staff following the Soham murders.
He recalled how he and David Blunkett, then Education Secretary, agreed in 1997 that school improvements should be about standards rather than structures.
"Less than a decade later we are once again mired in a debate which is essentially about structure," he said.
"That may be because the Prime Minister now believes that the only way to improve standards for all is to change the structure.
"If so, that flies in the face of the evidence of the past 50 years."
Even since 1997, improving school results overall does not lift the grades of the weakest children, he said.
The problem with developing what ministers have called a "radical new school system" is that it gives the impression that a lot is changing while distracting attention and energy from "the substance", he said.
"A perverse consequence of the current reform process could be that more young people end up dropping out because their needs cannot be accommodated in an educational mainstream so attuned
to raising the standards for the majority that it is unable to give adequate attention to those who struggle," he said.
As the row over the Bill nears its conclusion, it has also been suggested that Labour MPs are preparing to launch a new assault on selection in grammar schools.
Meanwhile, Birmingham Labour MP Lynne Jones has said she will not be swayed in her opposition to the reforms.
Dr Jones (Lab Selly Oak) refused to support the Government's Education Bill, despite a series of concessions made by Ministers.
She said the Government's key proposal - allowing businesses and other private bodies to help run schools - had not changed.
Concessions included tightening up a ban on selecting pupils by academic ability, by including legislation obliging schools to follow a code of practice.
Ministers hoped this would reassure backbenchers who feared the Government planned to create new grammar schools by allowing top colleges to select pupils.
But Dr Jones said she could not have confidence in the Government's promises because it had still not published the code of practice.