He warned of a "democratic deficit" within the Funding Agency for Schools - the quango charged with overseeing GM budgets, governing bodies and the opt-out voting system.
"A ballot took place at a secondary school in my area on a proposal put forward by a parent whose child left at the end of term," he told a conference organised by Local Schools Information (LSI).
"There is more to community interest than parents who happen to have children at the school. There is a democratic deficit here.
"The Government is going to be very reluctant to intervene because its policy will have been deemed to have been a failure."
The conference - Useful Lessons? A review of the grant-maintained experience - heard from opt-out head Jackie Kearns that GM schools were accountable to their local communities through their governing bodies.
She added: "It is optimistic theory that local education authorities are accountable." Mrs Kearns, head of Homewood school, Tenterden, Kent, and a member of the Funding Agency for Schools, said GM heads were also controlled by the national curriculum, rules on RE and worship, and the Office for Standards in Education.
John Wilkins, head of a GM school on the Stantonbury Campus in Milton Keynes, added: "Many people here would be very critical of the creation of new quangos which are not seen to be democratically accountable.
"But what is the distinction between a nationally elected government which wishes to appoint unelected people to a quango, and a democratically elected local authority which chooses to nominate to governing bodies people of its own choice?" Bob Lloyd, head of Hendon Boys, an opt-out school in the London borough of Barnet, said he would hate to go back to the days when governors were put on his school governing body by the LEA.
The conference, at the Royal Zoological Society in London, heard that within less than a year a whole cohort of pupils will have been through a grant-maintained school, yet the system has not been properly evaluated.
Gillian Shephard, the Education Secretary, told East of England GM heads earlier this month that exam results in opt-out schools were now better than those in their local authority counterparts.
But she admitted: "What we can't say yet is that they are doing so because they are grant-maintained." Michael Barber, professor of education at Keele University, called on her to commission research about the value-added difference the GM initiative had made to children's education.
But Jackie Kearns said: "The apparent anarchy of the Conservative Right has liberated schools to follow their own destiny and serve their children. The future development of this role is the responsibility of everyone who cares about the education of children. It is no longer a party political matter. "
It may be that its greatest impact has been on local authority schools, who have gained greater autonomy and had re-organisation proposals dropped by councils not wanting schools to opt out.
Local authorities have also had to re-think the way they operate. GM schools have undoubtedly gained financially, and used the money to buy extra equipment and books, and to take on additional staff.
Martin Rogers, from LSI, said opt-out schools were given Pounds 42.50 per pupil to spend on in-service training, while the amount allocated to maintained schools was Pounds 21 per pupil.
He claimed that GM schools were being double-funded, through central services compensation and financial regulations, by around Pounds 20 million. "A large majority of pupils in this country are being specifically discriminated against simply because their parents haven't chosen to go GM," he said.