Jeremy Corbyn is using day two of the election campaign to claim Tory education policy means children are being crammed "like sardines" into "super-sized" school classes.
The Labour leader claims Government figures show more than 40,000 primary school children in England were taught in classes of 36 or more last year, up from 38,500 in 2015.
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.
And Mr Corbyn claims there were 109 primary schools with more than 800 pupils in 2016, compared with just 16 in 2010 when the Tories came to power.
The Tories have immediately hit back, accusing Labour of a "massive own goal" because class sizes are rising in Wales, where Labour is in government.
But after tackling Theresa May on school funding in Prime Minister's Questions this week, Mr Corbyn says Conservative cuts to education budgets will mean even bigger classes.
"Seven years of Tory failure and broken promises have left our schools in a terrible state," claims the Labour leader.
"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.
"School leaders and teachers have said that Tory cuts to school budgets will mean class sizes will be forced to grow even larger. We cannot risk our children's education in this way."
Mr Corbyn claims that in secondary schools there were nearly 305,000 pupils in classes with more than 31 pupils, 18,000 of which were in classes with 36 or more.
Labour's shadow education secretary, Corbyn ally Angela Rayner, is attacking the Government for approving more than 130 new free schools this month.
And the Conservative general election manifesto is expected to contain more proposals for Mrs May's flagship policy of more grammar schools for England.
"This situation is becoming unsustainable - too many children are being taught in classes which are simply too big," says Ms Rayner.
"The system for school place planning is broken.
"The Tories need to let go of their unjustified fixation with free schools, but instead they have relaxed the rules so even more can be built in areas where there is no demand for places.
"Free schools are clearly not addressing the growing pressures on schools.
"Theresa May has no answers to the big challenges facing Britain, just more of the same failed policies which will take the country backwards and make us more unequal."
But a Conservative Party spokesman claimed: "This is a massive own goal from Labour. In Wales, where they administer schools with the Lib Dems, the numbers of infants in large classes has risen by 18% in just three years.
"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."