The warnings came as council-backed advisory group Local Schools Information said the plans would be challenged in court.
On Monday education secretary Gillian Shephard announced consultations on a new circular governing admissions policies which cut by half 'the forest of regulation and rigid bureaucracy' surrounding schools admissions arrangements. local education authorities will still control local admissions policies.
She proposes to give all schools the flexibility to select up to 15% of pupils by general ability or their aptitude in a particular subject. They will also be allowed to interview parents.
Mrs Shephard hopes her policy will be attractive to middle class parents and open up 'clear blue water' on education. Labour and the Liberal Democrats are strongly opposed to selective schools.
The Association of Metropolitan Authorities warned that the proposals opened the door to 'selection by social prejudice' and threatened 'the comprehensive principle that every child should have equal access to a good school within reach of their home'.
The Association of County Councils feared the changes would lead to 'sink' schools and accused the government of 'whittling away the right of parents to express a preference of school for their children'.
ACC education chair Saxon Spence did not believe schools would immediately press for more selection, but said there might be pressure in the long term for LEAs to allow it, as their schools saw grant-maintained schools following that route. 'LEA schools will get fidgety, and there will have to be negotiations,' she said.
Roy Jobson, chief education officer at Manchester City Council, did not expect any of his schools to press for selection.
'The opportunity for selection exists already and we have not had a single request,' he said.
Mrs Shephard's plans are open to legal challenge, according to LSI. It argues that changing selection procedures would amount to a significant change in the character of the school, which would breach legislation stipulating that such changes must receive ministerial approval.
There was certain to be a challenge from parents unable to get their child into their local school, LSI said.