The study, commissioned by Unison, found that 27 per cent of meals workers, 25 per cent of cleaners and caretakers, and 19 per cent of teaching assistants have at least two paid jobs. Of these, almost two-thirds would prefer a single job.
An NOP poll 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 on top of their contracted hours.
The vast majority are women and 50 per cent are on temporary or fixed-term contracts, rising to 80 per cent among teaching assistants. Unison says they are increasingly subsidising education.
In England, five unions have agreed to allow assistants to supervise whole classes from September 2005 but the National Union of Teachers plans strike ballots.