Negotiations to avoid industrial action over rising class sizes began this week as half a million children started school for the first time - at least 100,000 more than last year.
School representatives from the National Association of Schoolmasters Union of Women Teachers will be seeking meetings with their heads. If a solution cannot be found then ballots for industrial action can be expected by half-term.
The National Union of Teachers will also support action in schools with classes of more than 30 pupils.
More than a third of primary pupils are now in classes of 31 and over and Gillian Shephard, the Education and Employment Secretary, is coming under increasing pressure from parents and governors to find extra cash for schools.
The amount of money she secures in this autumn's public spending round could be vital for the Government in the struggle to regain the confidence of parents and teachers before the general election.
More than 1,000 NASUWT reps have been trained in class-size negotiation and studied their schools' timetables over the summer. Nigel de Gruchy, the union's general secretary, said larger classes would be tolerated if workload was reduced. "We do not want industrial action, but will use it as a last resort. "
Preliminary results from a union survey show members believe workload is less of a problem than previously although there are still "significant" difficulties, for example with report writing.
Governors estimate that 4,500 teaching jobs have now been lost because of cuts of Pounds 500 million in schools this year. The problems over class size are being exacerbated by rising rolls.
The Government is now planning to abolish the regulations governing the minimum space per pupil in schools.
The Association of Metropolitan Authorities will be meeting ministers later this month to discuss the proposal, currently out to consultation, which it believes will lead to overcrowding.
Alan Parker, its education officer, said the plan was monstrous and added: "The Government has completely ignored the enormous public concern about the general funding of education."
The National Governors' Council supported guidelines on the amount of space required per pupil and Margaret Tulloch, from the Campaign for State Education, said: "These regulations which the Government is planning to abolish are a safeguard for children."
The Department for Education and Employment is responsible for school premises - they are not covered by the Health and Safety Executive and, apart from the use of machinery in craft, design and technology lessons, are exempted from health and safety laws.
Earlier this year, the threat of a one-day strike by the NUT over class sizes at Poundswick junior in Manchester resulted in extra staff being appointed days before the action.
David Hart, general secretary of the National Association of Head Teachers, said heads had done their best to avoid excessive class sizes.
But he added: "They are the inevitable consequence of the Government's squeeze on public expenditure and school budgets. I sincerely hope that unions concentrate their fire on the true culprit - the Government." He said that while heads would discuss ways of reducing workload to make teaching large classes more tolerable, the room for manoeuvre was slight and only amounted to a sticking-plaster solution.
Mr Hart added: "Industrial action will not solve the problem. It is only by bringing home to the Government, together with parents and governors, the deplorable underfunding in the state sector and by making class size a political issue that there can be hope of reaching the cause of the problem. "