The majority of secondary schools in England have seen a rise in class sizes over the past two years, it has been suggested.
Union leaders argued that the increases were due to education funding cuts as they warned that larger classes mean less support for pupils and more pressure on teachers.
Ministers have insisted that more cash is being pumped into schools and announced a new funding formula which they say will ensure money is allocated in a fairer way.
The analysis, which draws on official figures, estimates that overall, 62% of England's secondary schools, for which comparable data is available, saw an increase in their average class sizes between 2014/15 and 2016/17.
It also indicates that out of 150 local authorities, 83% (125 councils) have seen a rise in average class sizes across their secondary schools, while 14% (21 councils) have seen a fall and 3% (four councils) have seen no change.
The analysis was conducted by the School Cuts coalition of unions - made up of the National Education Union (NEU), the National Association of Head Teachers (NAHT), the Association of School and College Leaders (ASCL), Unison, GMB and Unite.
ASCL general secretary Geoff Barton said: "We have repeatedly warned that schools have had to increase class sizes because of funding pressures and here is yet more evidence that this is the case.
"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."
Gail Cartmail, assistant general secretary at Unite, said: "The education and wellbeing of future generations is being harmed by cuts to school budgets.
"Schools across the country are being forced to cut staff, leading to less educational support for children and bigger class sizes.
"The Government should be investing in our schools to give young people the best start in life and the best chance possible to fulfil their potential."
Government figures show that the average secondary class size across England as of January last year was 20.8 pupils, compared to 20.1 at the same point in 2015 and 20.5 in 2012.
Last summer, former Education Secretary Justine Greening announced an extra £1.3 billion will be found for all of England's state schools from existing budgets, although some unions have suggested this will not be enough to plug funding gaps.
A Department for Education spokesperson said: "The union's figures are fundamentally misleading.
"We are investing an additional £1.3bn in schools funding over and above previous plans, so that spending on schools will rise from just under £41bn now to £43.5bn in 2019-20. As the independent Institute for Fiscal Studies has said, overall funding per pupil across the country will now be maintained in real terms up to 2020.
"We have also protected the base rate of funding for all 16-19 year old students until 2020.
"We have invested £5.8bn in school buildings, creating 735,000 places since 2010 and despite rising pupil numbers, the average class size has seen little change. In fact, less than one per cent of primary school pupils are taught in classes of 36 or more, a smaller percentage than in 2010."