Teachers have threatened Schools Secretary Ed Balls with national industrial action in a campaign to have their working hours cut.
The NASUWT union told Mr Balls it would ballot members with a view to staging a national "work-to-rule" from January 1 next year unless their workload eases.
Teachers at the union’s annual conference in Birmingham blamed "recalcitrant" and "arrogant" school heads for refusing to keep to Government rules aimed at freeing up their time.
NASUWT treasurer Sue Rogers told the conference that the deal unions struck with Ministers to cut their workload had been a success in many areas.
But she went on: "We want that success to be felt in every school. We want that success even if we have to drag every head, every governing body, every local authority representative kicking and screaming into obeying the law.
"NASUWT gives notice now that we will ballot our members for action in January 2009 to implement the agreement, to force it into schools unless the Government themselves urgently respond to the flagrant abuse of our statutory rights.
"The law is the law. Let us have the law."
Roger Kirk, seconding the motion, warned Ministers that teachers must see the difference in their working hours by December this year at the latest.
"It’s no good saying ’We have got a plan’. We want something to have happened. We have been waiting long enough. We are not prepared to wait any longer."
Under the deal struck between unions and the Government, teachers should have half a day off every week for planning lessons and marking while an army of classroom assistants have been hired to complete administrative tasks.
In a speech to the conference, Mr Balls said: "All teachers should get the benefit of their statutory entitlements.
"It is not right if teachers are being required to cover for absent colleagues during their planning and preparation time, if teachers are still invigilating external exams, or if they are still carrying out routine administrative tasks like chasing up pupil absence.
"If you are not getting your statutory rights, I want to know about it."