Support staff are increasingly used in the classroom
Teaching assistants are taking lessons alone in some schools in England, without being supervised by qualified teachers, a report says.
The study by London Metropolitan University said eight in 10 schools use teaching assistants in this way.
Assistants are meant to be support staff, but researchers found them covering for absent teachers.
Christine Blower of the National Union of Teachers (NUT) described the practice as a "dereliction of duty".
The report comes after teaching unions said bouncers and soldiers were being used for "crowd control" in some schools.
The government-backed study surveyed 1,764 head teachers, 3,216 teachers and 2,414 support staff.
In 40% of secondary schools, support staff were used to cover three-day-long teacher absences.
In some cases non-specialist staff took an entire term's worth of lessons.
The report said: "While in theory the cover supervisor's role was to supervise, most reported that they sometimes did more than this."
A third of support staff questioned said they regularly took classes alone.
They came from a range of backgrounds, including beauty salons, driving schools and working for the Post Office.
Christine Blower, general secretary of the NUT, said: "It is a dereliction of duty to put people who are not qualified teachers in front of children and expect that everything will be all right - it won't.
"Every child should have the right to a qualified teacher.
"Even in the present difficult climate the government has to provide sufficient funding to ensure that there are enough fully qualified teachers."
Schools Minister Vernon Coaker said: "We are absolutely clear that we want teachers in front of classes, not cover supervisors - parents would expect no less.
"It is the responsibility of heads to make sure that this good practice is maintained in their school."