Teachers could be denied leave for relatives' funerals and medical appointments under a new rule that specifies they will "rarely" have to cover for absent colleagues, The TES has learnt.
Members of England's social partnership between the Government, unions and employers have warned privately that there could be a downside to the cover rules being introduced in England and Wales in September.
Details of the workforce agreement, which was hailed as a breakthrough by some teachers' unions at its inception in 2003, were finally agreed last week after months of wrangling between teachers, heads and support staff unions.
The new rule means teachers will not be expected to provide cover for absent colleagues unless they have been specifically employed to do so or their school is operating under unusual circumstances.
Speaking at the Association of Teachers and Lecturers' Easter conference, Mary Bousted, the union's general secretary, said the new rule was a remarkable achievement.
But, in Wales, fears are growing that it could become a "financial headache" for schools.
At its annual conference last month, the Welsh-medium union UCAC warned it could "cripple" schools, unless properly funded. Elaine Edwards, the union's general secretary, said the rule could even lead to job losses. Gareth Jones, secretary of the head's union ASCL Cymru, said it could cost as much as pound;20,000 per school.
Official guidance says all schools should review their leave of absence policies as figures show that only 30 per cent of teachers' time off 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 spoke to officials from three separate social partnership organisations in England, each with differing views on cover. All agreed the change would mean fewer authorised absences for teachers. One admitted it might make a lot of teachers "less than happy" because time off for funerals or graduations could become a thing of the past.
It might also prompt a clampdown on absences for work-related activities such as school trips or examiners meetings.
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."
Another said schools might have to stop giving teachers time off for routine medical appointments or allowing them to leave early to pick up children.
The Government is poised to launch a consultation on the policy.