Two-thirds of secondary schools in England have increased the size of their classes in the past two years, an analysis of official figures suggests.
The analysis, by education unions, suggests the five areas with the fullest classes have all seen increases between 2014-15 and 2016-17.
In some areas, such as York, secondaries have had average rises of three more students per class, it said.
The Department for Education said the figures were flawed.
And it said average class sizes had seen little change since 2010, adding that in secondaries they were 20.8 per class in 2017.
But the unions, representing heads, teachers and support staff, say the fact classes are becoming fuller in the five areas with the fullest classes – Barnsley, Rutland, Thurrock, Newham and Leicester – shows ministers are failing in their stated aim to even out the differences in education.
Geoff Barton, general secretary of the Association of School and College Leaders, said it had repeatedly warned schools were having to increase class sizes because of funding pressures.
“It is the last thing they want to do but they have no other choice because they have to reduce staffing numbers and that inevitably affects the teacher-to-pupil ratio.
“Larger classes mean less individual support for students, and put more pressure on teachers at a time when we desperately need to reduce workload.”
Paul Whiteman, general secretary of National Association of Head Teachers, said: “The government’s own figures show that an extra 654,000 school places will be needed in England by 2026, to meet the 9% rise in pupil population.
He added “91% of schools face real-terms budget cuts compared to 2015-16 at a time when costs are rising and pupil numbers are growing”.
Kevin Courtney, joint general secretary of the National Education Union, said: “Time and again we are hearing about the consequences of the government’s inadequate funding of our schools and sixth-form colleges.
“Our children and young people deserve an education that is properly funded and resourced.
“For many it is their only chance of an education and it cannot be ruined by ministers who believe starving schools of cash is either acceptable or workable.”
The government added it was investing more than ever in schools with funds rising to £43.5bn by 2019-20.