Children are being crammed "like sardines" into "super-sized" school classes, Jeremy Corbyn has said.
The Labour leader highlighted an analysis of Department for Education figures which showed more than 40,000 primary school children were taught in classes of 36 or more pupils in 2016, up from around 38,500 the previous year.
A total of 16,655 primary pupils were being taught in class sizes of 40 or greater while nearly half a million were being taught in classes containing between 31 and 35 children, the figures showed.
'Proof of a school system in crisis'
There were also 109 primary schools with more than 800 pupils in 2016, compared with just 16 in 2010 when the Conservatives came to power.
Mr Corbyn said Conservative cuts to education budgets would only make the problem worse.
"Seven years of Tory failure and broken promises have left our schools in a terrible state," he said.
"Hundreds of thousands of our children are paying the price, crammed into classrooms like sardines.
"The prime minister herself has said that super-sized classes are proof of a school system in a crisis. And that's what we've got on the Tories' watch."
Mr Corbyn said that if Labour forms a government after the 8 June general election it would "stand up for all children by building a schools system for everyone, keeping class sizes down and making sure schools and teachers have the resources they need to ensure that every child, whatever their background, has access to a world-class education."
However, a Conservative spokesman claimed Labour had scored "a massive own goal".
Conservatives counter claims of complacency
"In Wales, where they administer schools with the Lib Dems, the numbers of infants in large classes has risen by 18 per cent in just three years," he said.
He added: "Of course we are not complacent about the situation in England. There is more to do and that's why we are spending a record amount on schools - something we can afford to do because of our careful management of the nation's finances."