Nearly half a million five, six and seven-year-olds were in classes of more than 30 at the beginning of the year. This month's class-size count - based on projections from almost 14,000 schools - shows that 345,000 pupils are in classes of 31 or more taught by one teacher.
The move was hailed by David Blunkett, the Education Secretary, as evidence that the Government meant what it said about cutting big classes.
The figures were greeted with scepticism by the Conservatives and some statistical analysts.
David Willetts, the shadow education and employment secretary, said: "I am a little bit suspicious of statistics that have been rushed out during Labour conference.
"In my experience Department for Education and Employment statistics usually run two years behind - now the DFEE is suddenly able to work out figures for September during September."
Local authorities have Pounds 22 million to spend over the next seven months on extra teachers - 1,527 have been recruited but not all will be full-timers.