Schools and colleges are being forced to cut courses and increase class sizes as a result of a growing funding "crisis", headteachers have warned.
Almost nine in ten heads and principals (88%) believe that financial pressures will have a detrimental impact on the education they are able to offer pupils over the next year, according to a survey by the Association of School and College Leaders (ASCL).
The findings, published as the union met for its annual conference in London, also show that the vast majority (81%) of those polled said they do not think they will have enough money over the next 12 months to meet the essential education needs of their school or college.
ASCL said that the growing funding issue is of "critical importance" to the education of England's young people and whoever is in power after the general election must take action to resolve it.
In total, three quarters (76%) of the 1,000 school and college leaders surveyed said they have not had enough funding over the last year.
Over half (57%) said they have had to cut the number of courses on offer, while a similar proportion (56%) have had to increase class sizes and around six in 10 (61%) have made cuts to resources such as IT equipment and books.
Just over a third (35%) said they have had to make redundancies.
Around one in seven (145) described their school or college's financial situation as "critical", with 35% saying it is "very serious" and 17% suggesting their financial situation is "serious".
Six in 10 (61%) said that financial pressures have had a detrimental effect on the education they are able to provide.
Looking ahead, more than half of those polled (53%) expect to make redundancies in the next year, with 67% looking at cutbacks to resources, the same proportion likely to reduce courses and 71% planning to increase class sizes.
Just under a third (31%) said their financial situation will become critical in the next 12 months, with a further 56% suggesting it will be serious or very serious.
ASCL said schools are facing a crisis as the public funding they receive has not kept pace with inflation over the last five years. Over the next 18 months, budgets will be further stretched by around a 4% to 5% increase in costs due to increases in pension and national insurance contributions as well as pay rises for staff.
General secretary Brian Lightman said: "This is not just a tight budget, we've had tight budgets for five years, this is a crisis." Schools are having to make "stark" decisions, he added.
Earlier this week, ASCL warned that schools are facing a "postcode lottery" in funding, with some likely to receive almost £2 million less than others over the next year.
It blamed the gulf on a "historic grant system that does not work" and called for a new, national fair funding formula to ensure schools are handed the money they need.
A Department for Education spokesman said: "As part of our plan for education, we have protected the schools budget and committed to introducing a national funding formula. We have also announced an extra £390 million for schools - the biggest step toward fairer schools funding in a decade.
"The Pupil Premium now worth £2.5 billion this year to schools - is also ensuring teachers continue to have the resources they need to give all pupils the best possible start at school, regardless of their background."
:: ASCL's poll questioned 1,026 school and college leaders in March.
Shadow education secretary Tristram Hunt told the conference he had heard their concerns about regional differences in school funding and the union's desire to move towards a national funding formula.
"I understand too that the complexion of educational inequality in this country is beginning to change quite dramatically," he said.
"The critical challenge is no longer east London - it is Suffolk, Hartlepool, the Isle of Wight and North Staffordshire.
"And, over time, the disparities in unequal funding have to be made much more equitable."