The survey, commissioned by Unison, the public-sector union, found that 53 per cent of administrators, 49 per cent of nursery nurses and 41 per cent of teaching assistants work up to 10 hours a week unpaid in excess of their contracted hours.
The vast majority of school support staff are women and about 50 per cent are on temporary or fixed-term contracts, rising to 80 per cent among teaching assistants.
The NOP survey found that 27 per cent of school meals workers, 25 per cent of school cleaners and caretakers and 19 per cent of teaching assistants had at least two paid jobs. Of these, almost two-thirds would prefer a single job.
A TESUnison survey last June found that most teaching assistants were paid between pound;7,000 and pound;9,000, although salaries varied from pound;7,125 in South Tyneside to nearly pound;18,000 in Gloucestershire.
Unison, which represents 60,000 teaching assistants, said that as their members took on more of the teachers' workload, support staff increasingly subsidised education.
Under proposals to reduce teachers' workload, eight unions have agreed to allow teaching assistants to supervise whole classes from September 2005.
Christina McAnea, Unison's national secretary for education services, said:
"The Government wants to enhance teaching assistants' role in education. If teaching assistants are in a more professional role, they have to be treated in a more professional manner. That means a decent wage, so that they do not need to take on two jobs."
Rosemary Plummer, 54, started work as a teaching assistant to fit in with her daughter's school hours.
Now her daughter Joanne is 33, but Mrs Plummer, Unison education convenor for Islington, is still working as a learning support assistant with special needs children in the north London borough.
She has done courses in the literacy and numeracy strategy and has a City and Guilds qualification. She says it is not fair that assistants are becoming more qualified but this is not reflected in their wages.
This week the National Union of Teachers, which did not sign the workload agreement, voted to stage strike ballots if it becomes necessary to stop teachers being replaced by teaching assistants.