Secondary heads were expecting a funding increase of more than 6 per cent per pupil next year after the Government said that the county council should increase its overall education funding by 6.8 per cent.
But they say they were told by the authority, at a meeting last month, that it was only proposing a 2.5 per cent per-pupil increase.
This is substantially lower than the minimum 4 per cent per-pupil increase guaranteed by Education Secretary Charles Clarke,in October, and raises the possibility of intervention.
Stephen Munday, head of Comberton Village college near Cambridge and chair of Cambridgeshire Secondary Heads finance sub group, said: "The authority says it can't afford any more because it can't get any more money from central government.
"But civil servants say it can afford it, but must prioritise school funding. That leaves Cambridgeshire schools not knowing where they stand."
Raymon Wilkinson, the Conservative-controlled council's cabinet member for education, said the authority had been placed in an impossible position. To fund services appropriately, including a 4 per cent per-pupil increase, would mean a 15.5 per cent council-tax hike. But the ministers have said it would cap double-figure rises.
Mr Munday said the council was proposing a real-terms cut on what had already been an "incredibly difficult" situation. Most schools in the county had a deficit budget in 20034, some as large as pound;750,000, he said.
"It would lead to staff redundancies and increased class sizes," he said.
"One secondary has already said it would not be able to operate a five-day week and others could be in the same position."