The Liberal Democrats have obtained figures from local education authorities representing more than half of England's schools which suggest that 7,740 schools will receive budget increases that fail to keep pace with rising costs.
Nick Clegg, the Liberal Democrat leader, said: "We believe that 2008 and 2009 are going to be the toughest years for school budgets for a decade, as rates of inflation exceed the money allocated to many schools. Early indications are that teacher redundancies will rise from the start of the new school year this September, impacting directly on teaching. For many schools, Blair's 'Education, Education, Education', is about to become Brown's 'Cuts, Cuts, Cuts'."
In a major education speech yesterday, he also said his party's research showed that funding for schools with the same proportion of pupils eligible for free school meals differed by as much as pound;500,000.
His team had used Freedom of Information Act requests to obtain information from more than a third of authorities, representing 10,457 schools - more than half of those in England. It showed that on average, budgets for 37 per cent of schools would rise by less than 3.1 per cent - the Government's own estimate of the increase in costs for schools.
Jane Bellamy, head of Wold Newton School, East Yorkshire, has already made one part-time and one full-time teacher redundant, partly because of budget problems.
"This a phenomenal problem for schools," she said. "Most have had a make-do-and-mend process for the last few years but we are reaching a point where we are past that".
Jim Knight, the schools minister, in January set out a minimum annual funding guarantee of 2.1 per cent for 2008-11, based on an assessment of cost pressures and an assumption that schools would find efficiency savings of 1 per cent a year.
Mr Clegg said his party planned to extend its pupil premium policy to benefit the children of parents on low incomes but in work. The policy would see pound;2.5bn extra going to schools attended by the country's 15 per cent most deprived pupils.
A spokesperson for the Department for Children, Schools and Families said Mr Clegg's predictions of cuts were nonsense:
"Local authorities have got average per pupil funding increases of 4.6 per cent this year. Funding is now at its highest ever levels in cash and real terms."