Ministers have put out for consultation proposals to abolish the provisions in the Education (School Premises) Regulations which govern space allocations, but can go ahead simply by laying the changes before Parliament.
Opponents believe one result would be to remove the minimum space requirements for private schools and nursery schools which might then get public funding under the voucher system or by opting in.
Margaret Tulloch, executive secretary of the Campaign for State Education, said: "Parents, teachers and governors will believe the regulations are a safeguard . . . and will be horrified to find that this change can go through without debate."
Groups condemning the move also include all the governor associations, the teacher trade unions and the National Confederation of Parent Teacher Associations.