The number of five to seven-year-olds being taught in illegally oversized classes has risen for the third year in succession.
Figures show that 22,080 pupils (1.6 per cent of the age group) are being taught in classes of more than 30 pupils - up 1,350 from 2004.
The figures show that the numbers have risen every year since ministers banned classes of more than 30 pupils in 2002. In that year, only 0.6 per cent of the age group were taught in larger classes.
The ban was the key education pledge of Labour's election campaign in 1997.
Headteachers' leaders blamed the increase on the Government's new workforce agreement with teachers - which entitles them to 10 per cent of time off from the classroom for marking and preparation.
Mick Brookes, general secretary of the National Association of Head Teachers, said schools did not have enough money to hire extra staff to cover classes during that time.
"The Government needs to properly think through these initiatives and properly fund them," he added.
"Unless you do that you are going to be taking funding out of the classroom for teachers and teaching assistants and that is inevitably going to damage education in this country." Steve Sinnott, general secretary of the National Union of Teachers, said the legal limit "must be enforced".
Nick Gibb, Conservative schools spokesman, said: "The Government promised that no children under the age of seven would be taught in class sizes of more than 30.
"Since this was one of Labour's key election pledges at the '97 general election, Ruth Kelly [the Education Secretary] needs to explain why they are failing to keep this promise."
But the figures do show that overall class sizes for five- to seven-year-olds have fallen from an average of 25.7 to 25.6 - although they have risen for older primary school children from 27.2 to 27.3
A spokesman for the Department for Education and Skills said: "Let's get this into context - we are talking about no more than an additional 40 classes out of 55,860. Much of this will be temporary. The funding is there to keep classes at a historically low level."
Yesterday's figures also show the overall number of teachers employed in state schools rose by 4,200 last year to 431, 900 - although it fell by 300 in primary schools largely because of falling pupil rolls.
Teaching vacancies also dropped by 140 to 2,480.
The percentage of ethnic minority pupils in schools rose from 18.3 per cent to 19.3 per cent - while the numbers entitled to free school meals fell from 17.3 per cent to 16.9 per cent.
The number of appeals by parents against the allocation of school places fell, too, from 64,000 to 59,700. Of these 20,600 were decided in the parents' favour compared to almost 21,400 in the previous year.Reuse content