Schools may have to redraw budgets for the next academic year after the Department for Education admitted it miscalculated its funding plans.
Because the number of pupils was underestimated, an original plan of a 2.7% increase per pupil in England for 2024/25 has now been revised to 1.9%.
The leader of the head teachers' union called the mistake "frustrating" for teachers planning for next year.
The government has ordered an inquiry and the department has apologised.
While the money had not yet been paid out, schools were given an indication of the funding they could expect to receive for 2024/25 in July, based on a national formula that determines how much each gets out of the £59.6bn schools budget.
But an update was published on Friday alongside an admission that the original version of the plans contained an incorrect estimate of pupil numbers.
In a letter to the education select committee, the Department for Education's top civil servant Susan Acland-Hood stressed the total schools budget would not be reduced.
But she said the amount promised to schools had to be recalculated because the department "uncovered an error made by DfE officials during the initial calculations".
The BBC has calculated that keeping to the originally planned increase of 2.7% per pupil would have meant the government having to find a further £370m to top up the overall schools budget.
The education secretary Gillian Keegan has ordered a "formal review...with independent scrutiny", the letter adds.
Geoff Barton, general secretary of the Association of School and College Leaders, said: "This is an extremely unfortunate and frustrating error.
"Even though schools have not received their 2024-25 funding, it is likely that trusts and local authorities will have used the incorrect figures in their budget planning and will now need to revise those budgets with the corrected figures.
"This is the last thing they need on top of all the other demands on their time."
Labour's shadow education secretary Bridget Phillipson said: "This staggering admission has revealed yet more Conservative-made chaos at the heart of the education system."
Union leader Daniel Kebede of the National Education Union said he was called in to an urgent meeting at the department this evening to discuss the situation.
The union said in a statement that the government is "not paying attention to the crisis in education", adding: "Head teachers have planned for that money and budgets are pared to the bone."
© OfficialAffairs
