A new rule meaning teachers will "rarely" have to cover for absent colleagues could also lead to them being denied time off for everything from relatives' funerals to medical appointments, The TES has learnt.
Members of the Government's social partnership with unions and employers are warning privately that there will be a downside for classroom teachers to the "rarely cover" rule being introduced in September.
The details of the new regime, publicly hailed as a breakthrough, were finally agreed last week after months of wrangling.
It will mean teachers will not be expected to provide cover unless they have been specifically employed to do so or their school has to deal with unforeseeable circumstances or staff absences outside the usual, historic pattern.
The change in teachers' contracts is seen as being the big gain for secondary teachers from the school workforce agreement, signed in 2003.
Speaking at the Association of Teachers and Lecturers' Easter conference, Mary Bousted, the union's general secretary, said: "This is a remarkable achievement and one that many thought would never happen."
But official guidance accompanying the change says all schools should review their leave of absence policies in the light of figures showing that only 30 per cent of teacher absence is down to illness. The other 70 per cent has been "authorised for reasons over which the school has had a measure of control".
The TES has spoken to officials from three social partnership organisations, with differing views on "rarely cover", and all agree the change will mean less authorised absence for teachers.
One admitted it might make a lot of teachers "less than happy" because time off for relatives' funerals or graduations could become a thing of the past.
The guidance was intended to "apply downward pressure" on the number of absences from timetabled lessons and might also prompt a clampdown on absences for work-related activities such as school trips or meetings with examiners.
"We have to reduce the need for cover if teachers are not being expected to provide it any more," they said.
Another official admitted: "It would be naive to assume that this will lead to anything other than a reduction in the number of absences granted by schools."
Some schools might even have to ask teachers to teach longer - say an hour a week - so that there was more money in the pot to pay for cover supervisers, one official predicted.
Another said schools might have to stop giving teachers time off for routine medical appointments or letting them leave early to pick up children.
"If an existing school absence policy is quite generous it may need to be looked at again," they said.
But Chris Keates, NASUWT general secretary, said teachers would not lose out and only unfair absence policies would go.
Whatever schools decide, "rarely cover" will be a practical headache, with many having to unpick cover budgets they worked out last term.
Social partnership unions have agreed to not comment publicly on the changes. All will have to conduct a full consultation with staff and unions on a new absence and cover policy before the end of term.