Leadership wants rank and file to support its ballot on possible strike action
the biggest teachers' union is calling on its members to vote for industrial action over heavy workloads.
The National Union of Teachers will write to members across England and Wales from Monday asking them to endorse school-based industrial action where negotiations fail.
Steve Sinnott, general secretary, said: "By responding positively to the ballot members will strengthen signicantly the union's campaign for a better worklife balance and improved conditions for all."
Senior union figures are nervous at government signals that it is turning its funding focus from schools to the health service.
In a letter to Alan Johnson, the Education Secretary, Mr Sinnott said that teachers are still faced with duties they believe are "unnecessary, excessive and unsupportive of teaching and learning".
By proposing industrial action, the union is drawing a line between itself and other unions that signed an agreement with the government on workload in 2003. While one NASUWT officer told its annual conference that "teachers have never had it so good", the NUT believes it, and other unions, are too cosy with ministers.
Next week's ballot is consultative. It would signal members' willingness to take action, rather than binding them to do so. Officials said action would be taken on a school by school basis.
NUT delegates passed a motion at their Easter conference, expressing concern at workload and its contribution to stress levels.
Hazel Danson, an NUT executive member and primary teacher in Huddersfield, said: "We are teachers. We don't walk off the job lightly, but there are enough schools out there suffering increased workload. By having this ballot behind them, they can demonstrate the union's willingness to escalate action if negotiations are unsuccessful."
Working hours of classroom teachers have reduced since 2000, but the decrease in the past year has not been significant. A recent survey by the School Teachers' Review Body showed that secondary heads were working much longer hours at 65 a week.
New NUT guidance notes that teachers are required to work only 32.4 hours a week, 39 weeks of the year - but the survey shows they are working 50.1 hours in primary schools, and 49.1 in secondaries.
Mr Sinnott said: "We don't sabre rattle. There may be schools where we will be forced to take industrial action, but we want to keep that number as low as possible."
Chris Keates, NASUWT general secretary, said her union had taken action in schools where workload cuts were not implemented. She said: "Although workload is moving in the right direction, there is still a lot more to be done."
The government says that guaranteed lesson preparation time and the reduction of administrative tasks had decreased workload.