Charles Clarke, the Education Secretary, was last night facing the prospect of strikes over redundancies by the teachers' union he has praised for its moderation.
The National Association of Schoolmasters Union of Women Teachers has told its members it will "respond vigorously" to any attempt to make teachers redundant because of the funding crisis.
Union leaders said this would mean balloting on strikes and sending an immediate national delegation to the schools concerned to demand redundancy threats be dropped. The union is also concerned by warnings that thousands of teachers who left this term to retire or take up a new post will not be replaced because of a funding shortfall.
Its leaders say they will also take action if this leads to remaining staff losing marking and preparation time - or any other increase in workload.
The union's tough stance is a blow to Mr Clarke as it has been one of his strongest supporters in negotiations over reducing teachers' workload through hiring extra classroom assistants to take over lessons so teachers can have 10 per cent of time away from the classroom to mark and prepare.
Eamonn O'Kane, general secretary of the NASUWT, said: "At a time when we're signing a national agreement on reducing workload, we should not end up with a situation where we are increasing the workload of teachers."
The union said it was surveying its branches to find out the extent of job losses. It says redundancies should be unnecessary because schools are holding an estimated £1.4bn in reserves that could be used to pay teachers' wages.
"The NASUWT is particularly concerned that the problems of funding are being quoted by some heads as reasons for not proceeding with the implementation of the national agreement [on reducing workload]," he said.
The NUT has voted for local and regional strikes if there are any redundancy threats.
Mr Clarke has told schools they can - as a one-off - use money allocated to them for building repairs this year to pay teachers' wages and avoid "needless redundancies".