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 claimed that 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 - who are often hired to take lessons while teachers are doing other non-contact work such as planning classes.
It found that more than a quarter (25.4%) of the teaching assistants and learning support workers questioned and almost half (49.1%) of higher level teaching assistants, who have more responsibility in the classroom, said they are asked to cover lessons.
Around 96.1% of cover supervisors said the same.
Of all of those surveyed, two-fifths (40.7%) said they had been asked to provide more or the same amount of cover for absent teachers in 2012/13 than the year before. The rest said they had been asked to provide less, or the question was not relevant.
In general, of more than 400 who said that they stand in for teachers when they are off more than two-thirds (69.4%) said they are asked to cover short-term absences of less than three days.
But 2.9% of teaching assistants, 1% of cover supervisors and 8.1% of higher level teaching assistants claimed they are asked to cover for longer absences.
One higher level teaching assistant at a secondary school in England said: "I prepare, teach and mark at least four lessons for two Year 7 bottom-set classes, and a Year 8 set for at least three hours a week. It is teaching on the cheap!"
And a teaching assistant at an English primary school said: "It is unfair that many TAs are teaching classes in the absence of a teacher, and doing the same job as a teacher for much less money."
ATL general secretary Dr Mary Bousted said: "Schools are selling children short by using teaching assistants to teach classes when the regular teacher is unavailable. We are totally opposed to this exploitation of support staff who are being used as a cheap option to teachers. It is grossly unfair on them and on the children and their parents who rightly expect their children to be taught by qualified teachers."
A Department for Education spokeswoman said: "The Government's recent review of school efficiency showed that, when properly trained and deployed, teaching assistants play an important role in helping to improve learning.
"But the rules are clear - they should not be teaching. It is for school leaders to use the expertise of all staff to ensure any disruption to pupils is minimal and that taxpayers get value for money."
:: The survey questioned 1,435 support staff members in state-funded schools in England, Wales, Northern Ireland and the Isle of Man.