
The figures show that 91 per cent or 17,942 schools will have seen or are due to experience a real-terms per pupil budget cut between 2015/16 and 2019/20.
The calculations are based on Department for Education data and were made for the School Cuts website, which is operated by the GMB, the National Association of Head Teachers, the National Education Union, Unison and Unite.
The unions say the latest analysis shows that 500 more schools face cuts than had initially been predicted.
The analysis currently estimates that schools have undergone £2.8bn of cuts since 2015 with the average primary school budget declining £54,000, and the average secondary school losing £205,600.
The unions are calling on Chancellor Philip Hammond to use this week's Budget to increase government spending on schools.
"Our recent survey showed that teachers are now paying for materials out of their own pockets to try and plug the gaps. This, however, is a crisis that goes far beyond a quick fix," said Kevin Courtney, joint general secretary of the National Education Union.
"The Chancellor needs to address this in his Budget by giving schools the money needed to ensure our children and young people get the education in the 21st century they both deserve and need."
During a Parliamentary debate on 6 November, Education Secretary Justine Greening accused the unions of "scaremongering" and misleading people with the figures released on the School Cuts website.
"Their claims are based on flawed calculations," she told MPs. "They say that money to schools is being cut when it is increasing."
The unions countered that the Department for Education's claims use the 2017/18 schools budget as a baseline instead of 2015/16, which the School Cuts calculations are based on. The government is also ignoring the impact of rising costs and inflation, the unions added.
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