Secondary schools could lose the equivalent of six teachers as a result of severe funding cuts, it has been suggested.
The average secondary in England is facing losses of almost £300,000, while primaries will lose out on tens of thousands of pounds, according to a new report by the Education Policy Institute (EPI).
It comes less than a week after teachers and parents in Kingston and Richmond protested Government funding cuts, with outer London boroughs set to be worst affected.
The report warns that growing financial pressures will mean that all state schools across the country are likely to see real terms cuts to per pupil funding in the next few years, with half of primaries and secondaries facing reductions of between 6% and 11% by 2019/20.
The average primary school will see a real terms drop in funding of £74,000 between 2016/17 and 2019/20, while the average secondary is set to lose out on £291,000.
These drops equate to the loss of two teachers for a primary school and six teachers for a secondary, it calculates.
The figures take into account financial pressures such as increases in pension and national insurance contributions, as well as cuts to a major education grant, and the impact of the Government's plans to introduce a new national funding formula (NFF).
EPI executive director and report co-author Natalie Perera said: "While the NFF is broadly welcome, our research highlights that the wider financial pressures on schools mean that all schools in England are set to experience real terms per pupil cuts in spending over the next three years, even after the new formula is introduced.
"Our findings suggest this could mean an average primary school loses funding equivalent to two teachers, while the average secondary school loses the equivalent of six teachers."
Under government proposals to introduce a new national funding formula, schools will get basic funding per pupil and additional money based on factors such as deprivation, and low prior attainment of pupils.
Ministers have said the new system will be fairer for schools.
The EPI report says that even though the Government is proposing to allocate more money to disadvantaged pupils as part of the formula, the overall impact of the reforms would be to shift funding away from the most disadvantaged youngsters towards those in the "just about managing" group.
And it warned: "The proposed increase to low prior attainment funding (from £1.4bn to £2.4bn), combined with plans to use the Early Years Foundation Stage (EYFS) as a measure of whether a child is at risk of falling behind, heightens the current incentive for teachers to depress pupils' results at the end of the Reception year in order to attract more funding to the school."
The EYFS assesses what a child can do at age five, at the end of their reception year at school.
Separately, two social mobility charities are warning that the new funding plans do not do enough to address the disadvantages faced by many poorer pupils living in deprived areas.
In its submission to the Government's NFF consultation, the Sutton Trust and the Education Endowment Foundation suggested that the new formula appeared to "advantage schools with low prior attainment ahead of schools with high deprivation".
A Department for Education spokeswoman said: "School funding is at its highest level on record at more than £40bn in 2016-17 and that is set to rise, as pupil numbers rise over the next two years to £42bn in 2019-20.
"Under our proposed new funding formula schools will be funded according to their pupils' needs, rather than by their postcode, with more than half set to receive a cash boost.
"Of course we recognise that schools are facing cost pressures, which is why we will continue to provide support to help them use their funding in cost effective ways.
"A recent report from the NAO outlines that schools should be able to make cost savings without affecting educational outcomes and our new School Buying Strategy is designed to help schools save over £1bn a year by 2019-20 on non-staff spend.
"We are consulting on the factors that will make up the formula and we know that it is important that we get this right so that every pound of the investment we make in education has the greatest impact.
"The consultation will run until March 22 2017, and we are keen to hear from as many schools, governors, local authorities and parents as possible."