David Blunkett, the Secretary of State for Education, said the figures showed the Government was on target to meet one of its key election pledges - the reduction of class sizes for all children aged seven and under by September 2001.
Conservatives immediately challenge ministers to prove that the reduction had not been achieved by increasing class sizes for older children or by restricting parental choice.
In January, ministers were embarrassed by figures which showed that class sizes were still rising.
But a survey of 146 of the 150 local education authorities to discover class-size estimates for mid-September suggests that the number of infants in classes of 31 or more is 345,000 compared with 485,000 last year.
Mr Blunkett said: "This is a product of investment worth pounds 22 million in revenue which has enable schools to recruit 1,527 extra teachers, together with pounds 40 million this year to new classrooms.
"These grants together with a further pounds 560 million over the next three years for new teachers and extra classrooms will ensure that we meet our class sizes pledge ahead of schedule.
"Parents will welcome these improvements, which will mean smaller classes, more teachers, more classrooms and the safeguarding of parental choice."
Most infants, he said, would be in classes of 30 or fewer by September 2000 and the pledge would be met for all infants by September 2001.
David Willetts, the shadow secretary of state for education, said: "We have always said that if you pursue the objective or reducing infant classes, you can certainly deliver it but at what price? How many parents will not be able to get their children into the school of their first choice because that would push class sizes above 30."
He said the Government must also publish figures about unsuccessful parental appeals on admissions and on the effect on class sizes elsewhere in primary schools. Yesterday's figures are a projection based on returns from 88 per cent of all primary schools.
Estimates published yesterday do not show class sizes for primary school children aged over seven, which also rose last year.
At the beginning of the year, there were 832,700 seven to eleven-year- olds, up by 53,100 on the year before.
Government sources insisted there was no evidence of an increase in class sizes for the eight to eleven age group.
But Mr Willetts said: "Ministers must publish figures which show where schools are reducing infant classes at the expense of more mixed-age classes, and bigger classes in later years."
Visiting schools around the country, Mr Willetts said he saw examples of both consequences of the Government's single-minded drive to reduce infant classes.Reuse content