Classroom teachers are working up to two hours a week longer than they were two years ago, but many are spending less time teaching, according to a survey by the School Teachers' Review Body.
The survey results, when compared with those of a similar one two years ago, show that primary classroom teachers are working on average two hours longer and secondary staff 1.4 hours longer.
The figures will put pressure on the Government to relax its push for a public-sector pay freeze in the case of teachers in recognition of the extra work forced on them by reforms and growing class sizes.
Heads and deputies in both sectors have not seen their hours increase since 1994, but secondary heads of departments are working, on average, 2.3 hours longer.
Between a third and quarter of staff say they do not always have the time to do their job properly.
Teachers do more than a quarter of their work at weekends and during the evening. Primary heads work 55.7 hours a week, their deputies 54.5 hours a week and classroom teachers 50.8 hours. Secondary heads work 61.7 hours, deputies 56.5, department heads 53 and classroom teachers 50.3. However one in ten primary heads is working up to 70 hours a week as is one in five secondary heads.
It is the younger and less experienced staff who are spending the most time teaching. As teachers are given extra responsibilities or are promoted, they have less and less to do with teaching children.
The STRB report says the average hours worked by primary classroom teachers tend to increase with class size, but not markedly except among the relatively small group with classes of 35 or more pupils. In primaries, the older the year group the longer the hours, and staff with mixed year or key stage groups work longer hours than those with only one.
Classroom teachers are spending some of their extra time teaching, but most of it is taken up in preparation, record-keeping, testing and assessment.
Headteachers, says the report, are spending relatively large amounts of time working with governors on financial management and on policy development. And despite schools being blamed for Britain's poor performance in the Olympics, the survey shows the time teachers spend on extra-curricular activities, such as sport and drama, is not falling.
The unions say that money is one reason for the rise in teachers' working hours. Heads are trying to keep schools afloat on less and staff are carrying out tasks, such as administration, which could be done by non-teachers. In the past two years more than 10,000 teaching jobs were lost, putting further pressure on class sizes.
Increasing administration is also to blame, says the National Association of Schoolmasters Union of Women Teachers. Brian Clegg, assistant secretary, who has conducted his own workload surveys, agrees that teachers are spending less time teaching children.
Nigel de Gruchy, the union's general secretary, says: "Far too much teachers' time is absorbed in activities which do not involve direct contact with pupils. Primary teachers spend hours tidying up their classrooms, a task that should be undertaken by ancillary workers.
"There is still considerable scope to slim down the administrative burden imposed by Government so that teachers can get on with the most important part of their job."
He said many teachers were forced to retire early because of the long hours and stress: "There are many harrowing stories of careers and lives ruined through overwork."
The unions will be using the STRB's survey to back their calls for statutory limits to the working week. The review body has so far shied away from recommending significant changes to teachers' working conditions or laying down limits on class size, believing these to be matters for school management.
Teachers are working longer than the 48-hour week of the European Union's social charter, says the NASUWT. And, says Brian Clegg, despite there being statutory limits on working hours in Scotland and Europe staff there are able to spend more time actually in the classroom than teachers in England and Wales.
The figures are based on teachers' workload during term time.