Government proposals to extend free nursery entitlement could be detrimental to the education of young children, a union has claimed.
Teachers will have less time to spend with individual children because resources will be spread more thinly, according to the National Union of Teachers (NUT).
A small-scale survey of NUT members working with young children found almost two thirds (62%) said they would have less time to track pupils' progress once the longer hours were introduced.
NUT members are also expected to demand a cut in their working hours, and to call for a day out of the classroom to mark and prepare work.
The union says that it is "alarmed" by the continued rises in teachers' working hours.
Most teachers still work more than 50 hours a week, the NUT said.
At the union's annual conference in Cardiff, NUT members are expected to consider a motion on workload and working time.
The motion calls for a further campaign to increase the amount of time teachers have for planning, preparation and assessment to 20% of timetabled teaching time. This is the same as a fifth of time, or a day away from the classroom each week.
Headteachers and other school leaders should get 60% of teaching time set aside for this purpose, it says.
The resolution also urges the union's executive to look at the impact of a 2001 agreement which introduced a 35-hour week for teachers in Scotland
