Unison says assistants are being used for cheap teaching across the UK.
Christine McAnea, of Unison, said the practice was "endemic" as it cost less to use support staff to cover teacher absence than to buy supply teachers.
Schools Minister Jim Knight said teaching assistants eased the burden on teachers, but they should not lead classes "for more than a short period".
It costs about £150 a day to employ a supply teacher, but about £50 to pay support staff.
Rosemary Plummer, a Unison representative, said in the last few months more than 40 teaching assistants from a small area of London had told her they felt they were being asked to do more than they were qualified for.
"They're delivering maths, they're delivering literacy and marking work - that's a teacher's job... they're being used as cut-price teachers," she said.
