More than half of secondary headteachers would ask staff to spend more hours preparing, teaching and marking lessons as school budgets tighten, a survey has revealed.
Some heads have already started saving money by making teachers spend more time in front of classes if their timetables allow it, in anticipation of expected budget reductions in March. Classroom unions say the move would be "short-sighted", making life even more stressful for overworked staff.
Heads who have already started to ask more of their staff have been criticised for taking action prematurely.
A survey by the Association of School and College Leaders (ASCL) revealed that 55 per cent of 400 heads questioned would "increase the contact ratio" of teachers to cut their staffing bills.
However, more than a quarter said their staff were already at full stretch and could not legally be asked to do more teaching hours.
A small proportion, 7 per cent, said they could not ask more of their teachers because it would "cause too much unrest".
Many heads have only marginal flexibility in their timetables because they are restricted by the Workforce Agreement, which gives teachers the right to 10 per cent planning, preparation and assessment (PPA) time.
The survey also revealed that just over a quarter of heads considered raising the pupil-to-teacher ratio to be the best way to reduce costs while minimising the impact on learning.
The news will strike fear into staff in schools where financial pressures are already beginning to bite and redundancies of positions such as advanced skills teacher have already been made.
Brian Lightman, general secretary of the ASCL, said increasing teaching hours where possible and increasing pupil-teacher ratios were a way of saving.
In September, Mr Lightman warned that some optional subjects such as languages and music could suffer as heads tighten their belts, trimming the curriculum down to its essentials to save cash.
He added: "These results show the difficult choices that heads are having to make. When you have to face a reduction in budgets, you have to find a way to make it work."
Allan Foulds, head of Cheltenham Bournside School in Gloucestershire, has already struck a deal with all teaching staff to teach one hour extra a fortnight, with the associated planning and marking.
He said: "There are not very many things you can do to reduce costs, and after consulting on this a year ago, we have done it.
"We would consider it again, but there comes a point where you can't do any more with it as you have PPA time and other things to consider.
"Nobody wanted to have the increase in contact hours, but there was a general acceptance it had to happen because Gloucestershire comes so low in the school funding league table."
He said in some schools it would be possible to increase class sizes without affecting performance. But he would be less likely to consider cutting back on support staff as this would affect some of the most vulnerable special needs pupils.
Mary Bousted, general secretary of education union the ATL, said increasing workloads was a "shortsighted" course of action.
She said: "Official surveys show teachers already work 50 hours a week. I'm not surprised making teachers work harder is being considered as an option, but it could result in more stress and illness through overwork and a subsequent drop in results."