Rules to protect teachers from covering their colleagues' lessons have failed to have a marked impact on working hours, a survey of 1,200 staff has revealed.
Workload has not dropped significantly since the "rarely cover" regulations were introduced last year, diaries kept by teachers have shown.
Staff praised the rules and dedicated planning, preparation and assessment (PPA) time as the two biggest factors in helping to keep workload under control.
But teachers are still working significant levels of overtime - secondary classroom teachers worked an average of 49.9 hours a week this year, half an hour less than a year ago, and the same as in 2008.
There was a greater impact for primary classroom teachers, who worked an average of an hour less this year compared to last - but they still work more than their secondary colleagues at 50.2 hours a week.
The Government-commissioned research comes just days after ministers announced plans for a major overhaul of teachers' pay and working conditions, which could see rarely cover and PPA under threat.
As part of a review to cut bureaucracy and hand power back to heads, education secretary Michael Gove is expected to water down or even abandon the workforce agreement, which dictates how teachers can be deployed by management.
Rarely cover, which was introduced in 2009 as the last piece of the agreement, is thought to be vulnerable as it has proved controversial with headteachers who struggle with the extra expense of bringing in supply teachers.
Unions have said that despite working hours remaining high, teachers are now carrying out more useful work.
Martin Freedman, head of pay and conditions at the Association of Teachers and Lecturers, said: "Teachers are working on average 2,000 hours a year, which is bloody hard and well over their 1,265 hours of directed time.
"But it does appear that teachers are being deployed more usefully, despite hours going up in some cases. If rarely cover or any other elements of the agreement go, it will be a retrograde step and teachers will be left covering admin."
Teachers in the annual workload survey - who kept diaries of the work for one week in March - also commented positively about being relieved of the 21 non-teaching tasks, such as bulk photocopying or ordering classroom equipment.
Chris Keates, general secretary of the NASUWT, and a supporter of the workforce agreement, said it would be wrong to hand back policies on working conditions to schools alone.
"If it's left down to local decision-making we will be back where we were 13 years ago, with a recruitment and retention crisis," she said. "You will find a very rapid decline if you get rid of the national framework. This government does not recognise a link between working conditions and improved standards."
She said critics of the national conditions framework who complained it was too restrictive were not aware of its built-in flexibilities which allow for local circumstances.
Christine Blower, general secretary of the NUT, said that rules such as rarely cover and PPA were beginning to have an impact on working hours.
"We will only continue to see improvements if enforceable limits and sufficient resources are applied to the problem," she said. "Teachers shouldn't have to pay for budget limits with their health."