Many education authorities are considering reorganising the academic year into five terms, instead of the current three, which would mean cutting the six-week summer break to four weeks and lengthening other holidays.
A national commission representing authorities in England is expected to recommend in a report in June that authorities switch to the new system.
Delegates attending the National Union of Schoolmasters/Union of Women Teachers' annual conference in Llandudno, however, yesterday voted unanimously to oppose the plans which they said were "change for change's sake". Teachers needed the long holiday to recuperate before the start of the autumn term, they argued.
Nigel de Gruchy, general secretary of the union, said later: "If they tried to impose it nationally we would have to respond on a national basis. We would consider anything up to and including strike action."
One tactic under discussion by the union is to stick with the traditional timetable and refuse to recognise changes to the school year. "If that means turning up to school when there are no kids there and not turning up when there are, so be it," Mr De Gruchy said. In Newham, east London, teachers staged a one-day strike earlier this year in protest at the proposals.
Supporters of the five-term year say the long summer holiday can lead to children forgetting much of what they learned during the previous year, and frequently use research based on similar practice in America to underline the educational advantages.
The NASUWT claims the research is flawed. Mr De Gruchy said: "We are very strongly opposed to the five-year term. It's all about parents wanting teachers to look after their kids during the summer holiday so they can go to work. That's the real agenda. If they introduced this, headteachers would go back in September more tired and exhausted than they already are."
Paul Mundt, a member of the union's national executive, told delegates on the final day of the conference: "All of this is change for change's sake. There is no logical research as to why we should do this. We all know we need an extended break at the end of the year to de-stress, wind down and look forward to getting away and out of school so we can survive."
Many city technology colleges already use the five-term year, although delegates complained that there was no evidence that it has boosted academic results. Other state schools, including Woodlands primary school in Grimsby, are operating the new-style year in pilot schemes. Oxfordshire will become the next authority to consider the changes when it launches a consultation process next month, delegates heard.
David Blunkett, the education secretary, has said he will leave it up to authorities to decide for themselves. But the national commission set up by the Local Government Association in January to look at the changes believes there is a strong case for reform. Its chairman, Christopher Price, a former Labour chairman of the Commons education select committee, has already warned that retaining the status quo is not an option.
Richard Hinton, a delegate from Cannock in Staffordshire, said teachers in his area had voted the plans down by a two-to-one majority. He said the five-term year was "like a disease" spreading through the country. "We have had the wonderful prospect of parents pushing their ideas forward to amend our working conditions. I find it deplorable."
Margaret Morrissey, of the National Confederation of Parent Teacher Associations, said she supported the long summer break, which gave parents the chance to spend more time with their children, but was disappointed by the threat of industrial action. She said: "Parents will be very unhappy about it because it is their children who will suffer."
