School leaders have voted to oppose reported plans to remove the cap on the number of hours teachers can be directed to work.
The NAHT school leaders' union debated an emergency motion after reports that the Department for Education has been exploring getting rid of the 1,265 hours cap.
The motion, which was passed unanimously, said that the plan would "add insult to injury" to teachers following the decision to freeze their pay.
Proposing the motion, Birmingham headteacher Amy Lassman said school leaders needed to send a message that "removing the cap on teacher hours was the wrong answer to the wrong question".
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She added: "We cannot use teachers to fill a gaping hole in the government's failing tuition policy...We should not use our expertise and experience to make bad plans work.
Removing cap on teachers' working hours 'would add insult to injury'
"We need to work on a better plan that meets the needs of our children and young people."
She described the reported proposal as a poorly thought-out idea that "reeks of disrepect for our profession".
The motion read: "Conference notes with alarm the reported proposal to remove the cap of 1,265 hours directed time from teachers' pay and conditions. In addition to the extension of the pay freeze, this adds insult to injury to the staff to whom we have a duty of care.
"Conference instructs the national executive to actively recognise the many, many hours of committed service teachers give beyond their directed time and to campaign for the government to drop any suggestion that teachers' directed time should be increased now or in the future.
"Conference further instructs the national executive to work with our sister unions in protecting the working conditions of our teachers."