Nearly four in ten schools are planning to reduce staff numbers over the next 12 months as headteachers struggle to balance their books, according to a survey of senior leaders.
A joint survey by heads' union the NAHT and The TES has revealed that 37 per cent of schools are expecting to see their staff headcount fall.
One "conservative" analysis suggests that this will see the overall national figure for school staff drop by over 17,000.
Almost 1,500 heads and senior staff responded to the poll, which also showed that 40 per cent of schools were anticipating a slump in their budgets - 5 per cent of which said they are bracing themselves for a cut of more than 10 per cent.
But it is the fall in staff numbers - through a combination of redundancy and "natural wastage" - that has been met with most concern.
NAHT general secretary Russell Hobby said the figure for job losses was conservative due to the number of smaller schools his union represents, and he warned that the cuts would be repeated for the next four years.
"The drop in the number of school staff may not seem that high, but we have to remember this is only in one year, and it is expected to get worse in following years," Mr Hobby said.
According to Brian Lightman, general secretary of the Association of School and College Leaders, the decline in staff will mean some schools will struggle to support the pupils who need it most.
"It is a very significant figure, and we have been warned that this is just the beginning" Mr Lightman said. "People are planning for a contraction and the only way to do this is through a reduction in staff."
The figures show that 81.9 per cent of respondents said their staff numbers will decrease through natural wastage, while 60.6 per cent said they are planning to reduce their workforce through redundancies.
John Morgan, headteacher of Conyers School in Stockton on Tees, said he will not be forced to let any staff go, but he said his budgets were "very tight".
"It is the added costs that make things very difficult," Mr Morgan said. "We still have to pay five-twelfths of the teachers' pay settlement, even though we haven't been funded for that by the DfE.
"On top of that there is the money we have lost from the devolved capital grant, which for our school was around pound;130,000 - that's a big loss," he added. "Then there are the extra services that schools will have to pay for on their own - it is going to become very difficult."
The news was greeted with dismay by union leaders, with public sector union Unison demanding the Government rethink its "savage cuts agenda".
Unison head of education Jon Richards said: "This survey nails the lie that schools are protected from the cuts. Cutting thousands of support staff at the same time as the number of pupils is rising will pile pressure onto remaining staff. This will hit the quality of education and put vulnerable children at risk."
NUT general secretary Christine Blower added: "This is yet another example of how hollow the coalition Government's words are about protecting schools' budgets. Many schools are already working with very limited resources.
"These cuts are a further devastating blow to education services and schools."
A DfE spokesman said: "We're increasing investment in schools by pound;3.6 billion over the next four years - protecting cash levels as well as putting money directly in heads' hands and cutting central bureaucracy to protect the front line."