Scotland's largest teachers' union has described the workload in schools as "unfair, unhealthy and unsustainable".
EIS general secretary Andrea Bradley said it was set to ballot members over strike action because the Scottish government had failed to reduce the amount of time teachers must spend in the classroom.
Almost three quarters of those who took part in a union survey said they were rarely or never able to do the work, preparation and correcting they had been asked to do within their working week.
The Scottish government said it was continuing to work with unions and local authorities on reducing the amount of class contact time for teachers.