New figures show the number of infant pupils being taught in classes of more than 30 children has grown by almost 150 per cent in Sheffield since the last election.
The situation is being highlighted by Labour candidate Oliver Coppard, who is taking on Deputy Prime Minister Nick Clegg in the Sheffield Hallam constituency next May.
Under Labour, no infant class could be larger than 30, but the rules were relaxed by the Coalition in 2012.
Now there are 1,500 more infants in Sheffield being taught in large classes than there were in 2010.
Mr Clegg says the claim is ‘rank hypocrisy’ at a time when massive investment is going into schools to meet demand for places caused by a continuing baby boom.
Mr Coppard, a governor at Firth Park academy, said soaring class sizes were yet another broken promise to Sheffield parents from Nick Clegg’s government.
“Such large class sizes are bad for children and bad for the teachers who need to teach them. People here were asked to vote for Nick Clegg in 2010 based on his promises to reduce class sizes.”
Nationally since 2012 the number of over 30 classes had grown by over 200 per cent, affecting 93,000 children.
But a spokesman for Mr Clegg said: “Under this Government we are seeing huge investment to deliver new school places in Sheffield.
“The last Labour Government cut funding for new schools places by 26 per cent nationally, despite warnings of a baby boom as far back as 2004.
“Just this month an extra 840 places have been created thanks to two new primary schools opening at a cost of £11 million, and plans are in the pipeline to build even more new schools to keep up with demand.
“Sheffield has seen £370 million invested to rebuild or refurbish every local school and this year local headteachers will benefit from £25 million in extra funding thanks to the Liberal Democrats’ pupil premium policy.”