Teachers are migrating in record numbers from local authority-maintained schools to academies and independent schools, new figures show.
State schools were advertising 23 per cent more vacancies than last year - the first indicator of new teacher shortages. John Howson, an education recruitment specialist, described these new shortages as "unexpected and worrying".
Class sizes are not shrinking as fast in state schools as in the independent sector, despite Gordon Brown's promise to close the pupil-funding gap between the sectors.
Figures show 6,500 state-educated youngsters were taught in classes of 31 or more at key stage 1, an increase of 50 per cent on last year. The National Union of Teachers called for ministers to cut primary-class sizes to 20 over 12 years.
Jim Knight, the schools minister, was criticised earlier this year when he described classes of 38 as "manageable", as long as teachers had sufficient support staff. The law requires key stage 1 class sizes to be limited to 30 pupils or fewer.
Mr Knight said this week that ministers were working with local authorities and schools to take action. "The law is absolutely clear on this and there can be no excuses," he said. "We expect local authorities and schools to take their legal responsibility to limit class sizes very seriously."
There are now 16.9 pupils for every teacher in local authority-maintained schools, according to the Department for Children, Schools and Families. That is an improvement of 8 per cent on 2001, but not as good as the 9 per cent improvement in independent schools, where each teacher has an average of only 9.6 pupils.
The number of full-time equivalent teachers in the independent sector rose to 52,430 this year, the Independent Schools Council reported. This includes a net gain of 1,519 teachers who moved across from the state sector last year.
In academies, full-time equivalent teacher numbers have increased by 2,490 in the past year. They now employ 5,820 teachers. These changes have affected the number of teachers in local authority-maintained secondary schools, which has dropped by 0.6 per cent to 215,400.