Class sizes are about to pass a 40 year high, one of the leaders of the country’s largest education union has said.

Kevin Courtney, joint general secretary of the NEU, will tell the union’s annual conference in Liverpool today: “What I’m going to reveal today about class sizes will be something we will come back to time and again in our funding work - because 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.

“We are forecasting that we are about to pass a 40 year high in secondary class sizes. Now remember these are average class sizes. And when these large-scale averages go up, if they aren’t planned for they can have even bigger consequences on peak class sizes.

“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.

“And bear in mind the number of secondary teachers teaching outside the subject they trained for.”