In a pamphlet published on Monday, Sean Williams, who was a member of the Number 10 policy unit under the Conservative government, says the reforms in the School Standards and Framework Bill will strip grant-maintained schools of funds and freedoms vital to their success.
He claims that the individual character of the London Oratory, the GM school attended by the Prime Minister's children, would be gravely weakened by the Bill.
The school's historic freedom to admit pupils from a wide geographical area would probably be restricted because its admissions arrangements would have to be approved by the local education authority every year and comply with a code of practice laid down by the education secretary.
Priority for local children would keep out those like Euan and Nicholas Blair, who have to travel from Islington to Westminster, Mr Williams says. He even suggests that the school might no longer be able to stipulate that its pupils be Roman Catholic.
The Bill abolishes grant-maintained schools but creates a new category - "foundation" status - for most of them with many of the same features. (Church schools such as the London Oratory that want to keep a majority of church governors will opt instead for "aided" status). Mr Williams says the changes will mean that GM schools lose money and independence, especially over staffing and admissions.
His pamphlet claims the Bill generally represents "a massive centralisation of powers" away from schools and into the hands of local authorities and the education secretary. It shows, he says, that "old Labour" rather than "new Labour" is in charge.
On staffing, he points out that the Bill gives local authorities the power to control numbers of all staff - including cleaners - at community and voluntary-controlled schools. For foundation and aided schools, the Bill includes 15 pages of legislation specifying every last detail about how to appoint staff.
And he criticises the clauses in the Bill requiring local authorities to prepare Education Development Plans for raising standards in their schools. "This represents a significant extension of the authority of the institutions ... which were responsible for such a large part of the failure of educational performance since the 1970s," he says.
'Levelling Down - The School Standards and Framework Bill' by Sean Williams is available from the Centre for Policy Studies, 57 Tufton Street, London SW1P 3QL (tel: 0171 222 4488), price pound;7.50.