The figures show the pressure schools are coming under, with heads expected to deliver efficiency savings worth 8% of their budgets by 2019/20.
A survey of 1,361 teachers, carried out by National Foundation for Educational Research on behalf of charity the Sutton Trust, found that 80% of secondary headteachers had cut back on either teachers or teaching assistants due to budget constraints.
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A third of primary school heads (32%) said they were using Pupil Premium cash to cover gaps, compared to 27% of primary school leaders.
“Our new polling adds to the growing evidence from highly credible sources that the squeeze on school budgets is having a detrimental effect on schools,” said Sir Peter Lampl, the chair of the Sutton Trust.
“Of particular concern is that schools are having to use funding for poorer pupils to plug gaps in their finances. Many are having to get rid of teachers to close these funding gaps.”
Today’s figures follow a recent report from the Public Accounts Committee, which said there was a “collective delusion” within the government about how spending cuts would affect school performance.
However ministers have pointed out that by 2020 per-pupil funding will be 50% higher than it was in 2000.
The government is also under pressure from MPs unhappy about the its new funding formula, which aims to equalise the amount spent on schools in different regions.
Although the respected Institute for Fiscal Studies has said the reform is “long overdue” and “broadly sensible”, schools in some areas are facing “protracted cuts”.
One of the most vocal Conservative critics of the plans, Geoffrey Clifton-Brown, said ministers would “have difficulty” getting the proposals through the House of Commons without making changes.