Class sizes in secondary schools in England will hit a 40-year high owing to a sharp increase in pupil numbers being compounded by the growing funding crisis, teachers’ leaders havesaid.

Kevin Courtney, the joint general secretary of the National Education Union (NEU), predicted that significantly more secondary school pupils would find themselves being taught in “super-sized classes”, which will “enrage” parents.

Secondary schools in some areas of England are already starting to experience significant growth in pupil numbers as a bulge in the population caused by rising birth rates in the 2000s makes its way through the education system.

School funding cuts are also having an impact. Last year education unions claimed that cuts were forcing up class sizes as schools sought to balance their budgets by cutting staff, with 62% of state secondary schools in England reporting larger class sizes than two years earlier.

In an address to delegates gathered for the final day of the NEU annual conference in Liverpool, Courtney accused the government of failing to prepare for the crisis, while class sizes continue to grow.

He told the conference: “We are about to see a significant increase in the number of secondary school children in super-sized classes. And my prediction is that this will enrage parents. The government can keep repeating its lines about more funding than ever before, but what’s happening to class sizes will give the lie to that.”

Courtney went on: “Astonishingly, we are about to see the number of secondary children in classes of 31 and above is about to pass the number in primary schools. That’s completely unprecedented in this country.”

Historically, classes in secondary school have been smaller than those in primary, but the rapid increase in secondary pupil numbers means that may change. There is also mounting concern that a recruitment and retention crisis means England’s secondary schools will not have enough teachers, particularly in subjects such as modern foreign languages, maths and science.

Many secondary teachers are already having to teach outside the subject they trained in – for example, PE teachers taking maths classes. “This represents a real problem in our schools,” said Courtney. “It represents a real problem for our members as well as their pupils. It’s a real problem this government has seen coming for 11 years. It’s a real problem it hasn’t dealt with.”

The secondary school-age population is forecast to grow by nearly 20% over the next decade. The government has been working to build capacity, but more than 10% of secondary school teachers left the profession last year. The government has also missed its targets for recruiting secondary teacher trainees for five years in a row.

Courtney also used his speech as a rallying call to urge parents to campaign on the issue of school funding in the run-up to the local elections, urging them to “ramp it up even more” in the case of a general election.

“Our funding campaign is going from strength to strength,” he said. “School funding is an issue that will not go away for this government … No politician in this country can safely ignore the fact that three-quarters of a million people changed their vote in the last general election because of school funding.’”

Teachers and school leaders were criticised in the rightwing media for their role in the campaign against funding cuts two years ago, before the 2017 general election, when education became the third most discussed issue on the doorstep.

The Department for Education has previously said that the government is on track to create a million new school places by the end of this decade. A DfE spokesperson later commented: “Since 2010 we have created 920,000 school places and secondary class sizes have remained low – around 21. This government is undertaking the biggest expansion in school places in two generations – and recent statistics show we are well on track to create one million places nationwide in the decade to 2020 – reversing a reduction of 100,000 school places between 2004 and 2010.”