The process required to rewrite the official guidelines on school admissions means new rules cannot be in place in time for the next school year. Most secondary schools will publish their arrangements for selecting pupils in the next few weeks and any late changes could leave them open to legal challenge from parents.
The Department for Education and Employment confirmed this week that the circular governing admissions is to be reviewed.
However, there is to be consultation on the changes and revisions of the regulations have to be laid in the House of Commons in the form of a statutory instrument.
The only change that can take effect immediately is the proposal to drop the requirement for GM schools to seek approval if they want to select up to 10 per cent of their intake on the basis of ability.
The more far-reaching proposal set out in the Prime Minister's speech is for GM schools to set their own admission criteria without approval from the Education and Employment Secretary.
According to Heather DuQuesnay, director of education in Hertfordshire and vice-president of the Society of Education Officers, such a free-for-all would make it very difficult for the education service to ensure coherent provision.
"If every individual (grant-maintained) school can decide its own formula, without even having to seek approval, then it will be impossible to provide for all pupils. They could all decide to select on the basis of academic ability or musical ability and not all children are academic or musical," she said.
In her view, any new guidance cannot be in operation before admissions in September 1997. "They may feel that they have fulfilled their promise by having the legislation in place. The lead time required could mean the changes would have to go ahead even if there was a change in government."
Others in the education service believe the proposals are a threat to parental choice. Chris Shires, deputy director of the London Borough of Hillingdon, said schools are currently required to apply objective criteria when selecting pupils.
"There are GM schools - though none in this borough - that require parents to be sympathetic to the aims and ethos of a school. That is unfair to parents because it is impossible to challenge any decision taken on subjective criteria. There is a danger of opening the flood-gates to admission policies that deny parents any choice because they cannot challenge the basis of the decision," he said.
The GM London Oratory - the school chosen by Labour leader Tony Blair for his son, Euan - selects pupils on the basis of an interview designed to elicit the extent of parents' commitment to the school's ethos. Such admission arrangements are forbidden by current DFEE guidelines.
They have not been applied retrospectively, but John McIntosh, head of the London Oratory and a policy adviser to the Conservative party, has complained that the DFEE will not allow him to apply the same admission policy to the new junior choir school that he is planning.
In his speech Mr Major said:"I see no reason why self-governing schools should not decide their own policy on oversubscription: nor about how you (grant-maintained schools) maintain the ethos of your school."
The views of the grant-maintained sector are to be sought and the DFEE has promised a consultation document.