School support staff are increasingly being asked to teach lessons, a poll has found.
It suggests although they may only be required to supervise classes, many workers feel they cannot do this without actually delivering lessons.
The Association of Teachers and Lecturers (ATL) survey also found the majority of staff are working beyond their contracted hours, and often do not get paid for the extra time they put in.
Overall, more than three quarters (77.6%) of the school support workers polled said they consider the work they do when acting as cover supervisor to be identical to that done by supply teachers.
This is up from 64% who said the same last year, ATL said.
And just over seven in 10 (72.5%) of those who responded to the question said it had not been possible to supervise a class without engaging in specified work - effectively delivering a lesson.
One primary school teaching assistant in Buckinghamshire told the union: “We are expected to deliver high-quality lessons not just supervise the class.”
And a higher-level teaching assistant (HTLA) working in a secondary school in Durham said: “We are told sometimes with only five minutes or less notice that we are covering lessons.
“We are expected to teach students.”
A minority, just over one in four (28.9%), said they are expected to carry out the full duties of a teacher, despite being paid at support staff rates.
Of those that said they work extra hours, around three in four (74.8%) said they do so because their workload demands it, the survey found.
Almost half (47.4%) said they work one to three hours over their contracted time, while 23.9% said they work four to six hours extra.
Around 11.5% said they work seven or more extra hours per week, and 16.9% said they do not work beyond their contracted hours.
Over seven in 10 (72.6%) said they do not get paid for working extra hours.
ATL general secretary Dr Mary Bousted, said: “As these results show, support staff are feeling the pressure to actually teach lessons and to plug the gap in staff shortages when teachers leave and do not get replaced.
“As the Government continues to squeeze school budgets, there simply aren’t enough funds to replace staff.
“It is worrying that this year more support staff feel the work they do when acting as cover supervisor is identical to that done by supply teachers, with an increase of 14%.
“Support staff are struggling under excessive workloads as much as teachers and this survey shows that, sadly, support staff feel over-utilised and undervalued.
* The ATL poll questioned 988 members working as support staff in state funded schools in England, Wales, Northern Ireland and the Isle of Man.