Schools have been accused of teaching on the cheap by using unqualified staff to take lessons.
Teaching assistants as well as cover supervisors are being drafted in to take classes, rather than bringing in fully qualified teachers, according to the Association of Teachers and Lecturers (ATL).
The union claims many support staff say they are being asked to provide cover for absent teachers, with some asked to take lessons for three days or longer.
It questioned more than 1,400 of its school support members, including teaching assistants, higher level teaching assistants and cover supervisors and found that more than a quarter (25.4 per cent) of the teaching assistants and learning support workers questioned and almost half (49.1 per cent) of higher level teaching assistants, who have more responsibility in the classroom, said they are asked to cover lessons. Around 96.1 per cent of cover supervisors said the same.