Britain's biggest teachers' union is threatening nationwide strike action in protest over large class sizes if ministers fail to bring in a new legal limit of no more than 20 pupils per class.
Leaders of the National Union of Teachers warned of imminent industrial action if they fail to convince the Government to do a U-turn over its stance that there should be no more legislation to reduce class sizes.
It means thousands of children throughout the country could be sent home from school.
The threat of national disruption – the first delivered by the union since Labour was elected in 1997 – represents a low ebb in relations between the union and the Government.
The NUT is also balloting for strike action over pay with a view to staging a national stoppage on 24 April – in protest over a 2.45per cent pay offer.
An emergency motion to be debated today threatens action on class sizes and pay. Teachers will demand the new legal limit on class sizes should be in force by 2020.
If the Government fails to agree to back legislation – as the Schools minister Jim Knight indicated would be the case in an interview on Radio 4's Today programme, NUT members say national action could follow within the next year.
Mr Knight drew criticism when, at the Association of Teachers and Lecturers' conference, he said he had seen a class of 70 pupils – with one teacher and three classroom assistants – work perfectly well.
Earlier, he had been jeered by delegates for failing to condemn a class size of 38 in a Wakefield primary school in Yorkshire.
Yesterday Steve Sinnott, general secretary of the National Union of Teachers, said he thought the class of 70 pupils was "an unacceptable teaching arrangement for anything other than exceptional circumstances".
"The youngsters have to have their work marked and get feedback on what they have done. That has to be done by the teacher – that can't be done by the other adults in the classroom," he added.
"That's why the argument Jim Knight should be making is that this is unacceptable and I'm not surprised there was an outcry over his comments at ATL. There would be an outcry at the NUT conference and an outcry in every staff room in the land.
Teachers' leaders want the maximum class size introduced as part of Gordon Brown's avowed commitment to bring spending on state schools up to the level of that in independent schools.
So far no target date has been set for that with officials describing it as an "aspiration" rather than a target.
At present, the average class size in a primary class is 26 – one of the highest in the Western world, according to a survey by the Organisation for Economic Co-operation and Development. In secondary schools, it is 21. In private schools, the average is near 14 in secondaries and 12 in prep schools.
The only legal limit on class sizes at present is 30 for five to seven-year-olds – introduced by Labour in 2002.
On pay, Mr Sinnott said the 2.45 per cent offer was "a pay cut" as inflation was running at about 4 per cent. He warned that newly-qualified teachers were "struggling to make ends meet with debts of around £20,000 when they start work".