Teacher shortages mean classroom support staff are doing work normally done by qualified teachers, suggests a survey of union members.
Some two-thirds of almost 700 classroom supervisors who responded felt they did the work of supply teachers, says the Association of Teachers and Lecturers.
"This can only lead to lower educational standards," said ATL general secretary Mary Bousted.
The government said support staff should not be used to replace teachers.
Last week, a report by the official spending watchdog confirmed there were growing teacher shortages in many areas, with the government having missed recruitment targets for four years.
Dr Bousted said teacher shortages, along with increasing pressure on school budgets, were leading to the use of support staff to replace teachers "in a way that was never intended".
"So rather than hiring a temporary teacher, schools are asking support staff to teach.
"They should not be asked to do it," said Dr Bousted.