One in four classroom teachers is working more than 55 hours a week with little prospect of new conditions of employment.
The School Teachers' Review Body has ruled out nationally prescribed staffing levels despite discovering that work levels in schools were increasing.
A workload survey by the review body disclosed classroom teachers were working up to two hours a week longer than they were in 1994, and that last year 2 per cent of primary heads and 15 per cent of secondary heads were putting in 70-plus hours a week.
The four main teacher unions have been pushing for minimum staffing levels and claim that the STRB's insistence that problems were best sorted out locally was not working.
They had wanted new limits on overall working, but suggested no figures to alter the 1,265 hours teachers are required to work in the school year.
The STRB remained unconvinced change was necessary and said in a statement: "We do not think it appropriate to have a detailed framework on how teachers spend their time." It said minimum staffing levels would limit the ability of schools to make the best use of their budgets. But it warned: "The success of the system also depends on a balance being struck so that the willingness of professionals to respond flexibly to unexpected and sometimes unwelcome demands is not overstretched: once, lost, such willingness may not be easy to re-establish."
According to the STRB survey, which was carried out in March, primary classroom teachers worked on average 50.8 hours a week.
The length of their working week increased as class sizes grew, with teachers working with classes of 35 or more putting in 52.9 hours a week. The older the year group the longer the working week.
The STRB report showed that the average working week for secondary teachers was slightly less - 50.3 hours.
Maths and English teachers had the heaviest workload (50.5 hours and 51. 5 hours a week respectively), younger teachers, those under the age of 25, also put in long hours (52.1 hours a week).
Examination of the results from 1994's workload survey showed big rises, particularly in primary schools, in the number of teachers who worked more than 55 hours a week.
In 1994, some 17 per cent of primary teachers worked more than 55 hours a week. Last year, that figure had increased to 27 per cent. The number of such secondary teachers rose from 20 per cent to 25.2 per cent.
Meanwhile, the proportion of teachers working between 40 and 55 hours a week dropped. It fell from 72 per cent of primary teachers in 1995 to 67 per cent last year, and from 12 per cent in secondaries to 9.1 per cent.
Research by the STRB showed primary teachers spent more time on lesson preparation and marking than their secondary colleagues.
According to the review body, primary teachers spent almost a third of their time (32.3 per cent) on lesson preparation and marking, compared to the 29.8 per cent spent by secondary teachers.
Primary teachers spent 20 hours - or 39.3 per cent of their working week - teaching, while secondary teachers were in class for 20.9 hours, or 41. 5 per cent of the week.
The pay review body said primary heads worked a 55.7-hour week and secondary heads worked 61.7 hours.