Education unions have revised upwards their predictions for the number of schools facing funding cuts to almost 18,000.

The coalition of organisations behind the popular School Cuts website say 557 more schools face real-terms cuts in their funding by 2019 than originally thought.

The website, run by the National Education Union, the National Association of Head Teachers and support staff unions GMB, UNISON and Unite, now predicts 17,942, or nine in ten schools, will be worse off by 2019 compared to 2015.

In September, it was predicted that 17,385 schools would face real-terms over the four years to 2019, despite an additional £1.3 billion going into school funding over the next two years.

While the extra £1.3bn is welcome, it is short of what schools need if standards are to be preserved

But after obtaining more up-to-date figures, including exact pupil numbers for every school in England, the unions have revised the figure upwards.

The School Cuts website, which receives 20,000 hits a day and is widely credited with raising awareness of school funding problems during this year’s general election campaign, has been the subject of increasingly strong criticism from the government in recent months.

The website uses the government’s own figures to calculate how upcoming funding changes, including a new funding formula, will affect budgets in each school.

But its conclusions were branded as inaccurate “scaremongering” by Justine Greening, who last week told Parliament last week that “the actual funding” for schools could be found on the government’s own ready reckoners, as those must comply with Office for National Statistics standards, “unlike some of the websites that put up inaccurate data”.

“They say that money to schools is being cut when it is increasing, and they say that teacher numbers will go down although they are going to go up,” she said.

The amount schools will receive in cash terms is due to increase over the next two years, but it will increase at such a low level it amounts to an average real-terms cut of 4.6 per cent when increased cost pressures are taken into account, according to the Institute for Fiscal Studies.

Schools are facing greater pension and national insurance costs, plus additional pressures from the apprenticeship levy, mean the majority of schools stand to lose more than they will gain.

The unions say they are being “completely open about their figures and methodology”, and have called for additional investment in schools in next week’s budget by the Chancellor of the Exchequer.

“Many other groups, including parents, school governors, and a delegation of more than 5,000 head teachers have been arguing for some time that school budgets are at breaking point. The school funding crisis is real. Funding increases need to be in real terms, not in cash terms,” a spokesperson said.

“While the extra £1.3 billion, which the DfE is reallocating from its existing budget, is welcome, it is short of what schools need if standards are to be preserved.”

A government spokesperson said the School Cuts campaign’s claim was based on a “flawed calculation” because it started from the position of school budgets in 2015-16, and then “calculates the cost pressures on school budgets over 4 years”.

This did not reflect that most of the pressures “have already been absorbed by schools, at the same time as standards have continued to rise”, the government said.