A Labour government will limit primary school classes in England to no more than 30 pupils if the party win the general election in May, Ed Miliband pledged.
The Labour leader said the party would create thousands of new school places, including in over-subscribed schools, enabling it to cap class sizes for five, six and seven-year-olds.
However, the Conservatives said primary school classes were already capped at 30, and that Labour's proposals would mean getting rid of "sensible exceptions" designed to ensure the children of armed forces personnel got places and that twins were not split up.
Mr Miliband's intervention comes as Nick Clegg is promising to put education at the centre of the Liberal Democrats' general election campaign.
The previous Labour government originally introduced legislation in the 1990s limiting primary school class sizes to a maximum of 30, although the rules have been relaxed under the coalition.