Schools could face a bill of more than pound;800 million to settle equal pay disputes for support staff, new figures suggest.
The impact on budgets could force schools without large reserves to make redundancies, teaching unions have warned.
Every local authority in England has to carry out a pay review to rectify years of underpayment to female workers.
Around 29 per cent of the estimated total bill of pound;2.8 billion applies to support staff in schools, a survey by the Local Government Employers has found. In extreme cases, school staff account for up to 75 per cent of local authority costs.
Fears have been raised that school budgets will be hit as councils ask schools to use their reserves to meet the bill.
Schools hold a total of pound;1.6 billion in their reserves, but many have committed money to capital and other projects.
Kerry George, senior assistant secretary for salaries, pensions and conditions at the National Association of Head Teachers, said local authorities should settle the pay review.
"This should have been dealt with 10 years ago and to expect schools to take a hit is outrageous," she said.
"We don't want to be alarmist, but redundancies have to be a possibility where schools do not have uncommitted reserves."
In Sandwell, schools were told last year that they would have to pick up a pound;55 million bill in a worst case scenario if negotiations with unions failed to reach a deal for eligible staff.
Jan Parkinson, managing director of the Local Government Employers, said it was right that schools paid.
"In the circumstances, it seems a fair balance of responsibilities if councils take care of the legal and industrial relations issues and schools pay for back pay from their considerable reserves," she said.
Attempts to reach pay settlements are being hampered by no-win no-fee lawyers, Ms Parkinson said.
Lawyers are taking up the cases of individual workers who have received less in group settlements than the six years of backpay they are entitled to.