Teaching assistants are increasingly being asked to give lessons, a poll has found.
It suggests although school support staff 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 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 per cent who said the same last year, ATL said.
One primary school teaching assistant in Buckinghamshire told the union: "We are expected to deliver high-quality lessons not just supervise the class."
A minority, just over one in four, 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, said they do so because their workload demands it, the survey found.
More than seven in 10 said they do not get paid for working extra hours.
ATL general secretary Dr Mary Bousted, said: "As the Government continues to squeeze school budgets, there simply aren't enough funds to replace staff."
She added: "Support staff are struggling under excessive workloads as much as teachers and this survey shows that, sadly, support staff feel over-utilised and undervalued.
A Department for Education spokesman said: "Teaching assistants are allowed to teach classes but we are absolutely clear that schools should use them to add value to what teachers do, not replace them. It is up to individual schools to decide how to train, develop and use their teaching assistants effectively.
"We want schools to have the resources they need, and through our careful management of the economy we have been able to protect the core schools budget in real terms. That means that in 2016-17 schools have more funding than ever before for children's education, totalling over £40 billion. The national funding formula we are currently consulting on will ensure schools are funded according to their pupils' needs, rather than by their postcode, giving headteachers certainty over their future budgets and helping them make long-term plans."
The poll comes amid continuing concerns about a growing teacher shortage in England , especially in disadvantaged schools and subjects such as physics.
In addition, a recent investigation by the National Association of Headteachers (NAHT) found nearly eight in 10 vacant posts were considered ''difficult to recruit to'', while more than one in six went unfilled.