Teachers could be the first thing to be axed in schools when widely forecast cuts to education spending are introduced, a survey has found.
Nearly two-thirds of secondary heads said they would consider increasing class sizes as a "priority measure" if they lost 2 per cent of their budgets.
Nearly half of the 200 heads asked said they would also consider cutting back on textbooks and other subject resources, limit subject choices post- 16 and resist spending on new ICT equipment.
The same proportion also said they would look at increasing staff teaching loads and cutting back on staff training to plug the holes in the budget.
The survey came as headteachers' associations warn that reining in education funding could "destroy" the improvements made over almost a decade of record-breaking spending.
Although current levels of cash look to be assured until 2011, ministers have promised a "real-terms" increase of just 0.7 per cent in pre-16 and 0.9 per cent in post-16 education for 20112012.
Heads and funding experts fear that increased participation in early years and post-16, coupled with a promised 1 per cent increase in National Insurance in 2011, could swallow up the extra cash and actually amount to a funding cut.
The Association of School and College Leaders (ASCL), which carried out the survey, has already advised members to tighten their staffing budgets for next October, after schools were asked to make 1 per cent "efficiency savings" this year.
Nigel Blackburn, executive head of Hayesbrook School in Tonbridge and the Wildernesse School in Sevenoaks, Kent, said making staff savings was incredibly difficult because of the demands of the Workforce Agreement and Ofsted.
"Fortunately we have many staff going on maternity leave and most want to work part-time, so this is a way of cutting back without having to impose it on anyone," he said.
John Morgan, president of ASCL, said the Government needed to take a "long-term strategic view" of the country's needs.
Speaking to The TES before his keynote speech to the association's annual conference in London today, he said:
"If there are real-terms cuts there is no way we are going to be able to compete with emerging economies like India and China.
"We will be competing with them in terms of creativity and inventiveness, the things that come from a high-quality education.
"We have an education system that's the envy of the world for the creative minds it produces.
"We might not be quite as efficient as the Finns, but they don't have a single university in the top 100 and we have 15.
"We realise we won't be getting what we have been getting, the major increases we've seen for the past 10 years aren't going to carry on, but we need to at least maintain the level that we have reached."
He said Britain was "regarded worldwide" for its imagination in the classroom and training of teachers and leaders, and could not afford to throw that away.
He also warned that budget cutting would not come naturally to today's leadership teams, many of whom may never have been called to do it before.
Mr Morgan, who is also on the board of the university admissions authority UCAS, was also expected to criticise recent cuts to university education and several elements of the new Children Schools and Families Bill, which has just reached the House of Lords.
He will tell delegates that the "pupil and parent guarantee" and the "licence to practise" are "unnecessary bits of bureaucracy".