Almost 1 million school pupils in are in classes of 31 or more, a surge of nearly 30% since 2010, according to a report.
Data from the National Education Union also shows that nearly 20,000 more pupils were in supersized classes of more than 36 in 2018-19, compared with eight years ago.
Mary Bousted, the joint general secretary of the NEU, blamed the figures on a “real-terms funding crisis” in education.
Most constituencies in England have experienced an increase in average class sizes since 2010, the report suggests. Class sizes increased on average in 474 out of 533 constituencies, and fell in just 59.
The Department for Education said last month that average class sizes had remained stable.
A total of 961,127 pupils in England were taught in classes of 31 or more in the academic year 2018-19, a 29% rise from 2010-11. The percentage of students in classes of 36 or more rose by 44% in the same time frame, to 63,566 pupils in 2018-19.
The most dramatic development has been faced by secondary pupils, of whom 21,843 were sitting in classes of 36 or more in 2018-19, a 258% increase since 2010.
The NEU said 34% of teachers had declared a reduction in class size as their “absolute top priority”, regardless of who enters No 10.
Pupils were experiencing “the inevitable result of several government policies which have conspired to put a squeeze on schools”, said Bousted. “The real-terms funding crisis has had catastrophic effects, including a direct impact on class size.”
“Today’s analysis will ring true for every parent who has witnessed their school cutting teaching assistant posts, reducing subject choice or organising fundraiser events and begging letters.
“Parents are no fools. They can see with their own eyes the impact of funding pressures on their children’s education and the reduction in individual contact time that their child has with their teachers.”
David Morris, the Conservative candidate for Morecambe and Lunesdale, described the report as “scaremongering”.
He said: “Class sizes remain stable while standards continue to rise. Since 2010, we have created 921,000 new school places. Thanks in part to our reforms, 1.9 million more children are being educated in good or outstanding schools, so more children are getting the best start in life.”
Geoff Barton, the general secretary of the Association of School and College Leaders, said: “We have been warning for some time that class sizes are rising as a result of severe funding pressures and these figures show the chickens have come home to roost.
“The equation is simple: if schools have less money, they can afford fewer staff, and larger classes are an inevitability. Large classes make it more difficult to provide individual support to pupils and they are harder to manage.
“The current government has promised more funding but it does not go far enough. If we are to tackle this issue we must have a commitment which reverses the cuts in full and ensures that funding in the future rises at least in line with actual school costs.”
A study showed recently that Britain has the biggest primary school classes in the developed world, with an average of 28 pupils.