The number of children aged five to seven in classes of more than 30 has increased by a fifth to 29,000 in the past 12 months, prompting claims that the Government has reneged on its 1997 manifesto pledge that all infants would be taught in classes of 30 or fewer.
Ministers defended the increase, saying fewer schools than last year are breaking the law but that more are using temporary exemptions intended to help pupils who move to an area and those with special needs.
Average primary class sizes rose to 26.3 pupils, despite an increase in the number of primary and nursery teachers and a continuing fall in pupil numbers. This compared to an average of 26.2 in 2005, but remains below the 1997 figure of 27.5 pupils.
Infant class sizes (25.6) were significantly smaller than those at junior level (27.3). Headteachers said the introduction of guaranteed non-contact time for teachers was partly responsible for driving up class sizes.
From the beginning of this school year, all teachers were entitled to spend half a day a week out of the classroom marking and preparing lessons - so-called "planning, preparation and assessment time" (PPA).
Mick Brookes, general secretary of the National Association of Head Teachers, said: "Some of our members have said that the only way they can deliver PPA time was to release a member of staff. That will push class sizes up. Funding has not been there to produce what schools think they need, which in most cases is a teacher in front of their children."
Secondary classes fell to an average of 21.5 pupils in January 2006, compared to 21.7 last year, helped by an additional 900 teachers.
The figures show the number of teachers has increased by 3,500, or 9 per cent, since Labour came to power in 1997. There are now 435,400 full-time equivalent teachers in state schools.
In the same period, the number of support staff has more than doubled, to 287,100, with a 22,300 increase in the past 12 months.
Jacqui Smith, schools minister, said: "More teachers and support staff are evidence of the positive impact that our record investment is making on standards in our schools. We have made significant progress in meeting our commitment to limit the class sizes for five, six and seven-year-olds. This year the number of classes that contravene class-size regulations has fallen.
"However, there has been a small rise in the number of classes - with valid exceptions - where numbers exceed 30 children."
The Association of Teachers and Lecturers said the figures showed the need to recruit more teachers from ethnic minorities. Fewer than one in 10 teachers was from an ethnic minority, compared to almost one in five pupils. One in eight primary pupils and almost one in 10 secondary pupils did not speak English as their first language.
There was little change in the time teachers spent off work due to sickness, the figures reveal, with each member of staff missing an average of 5.2 days in 2005, compared to 5.3 days in 2004.