Nearly half of supply teachers say they have been offered less work in the past year, a new report reveals.
The poll by the National Union of Teachers also revealed that supply staff were nearly four times more likely to be physically threatened by pupils and that less than a third had been offered any training in the past year.
Of the 46 per cent of respondents reporting less work, 31 per cent blamed school funding problems - supply teachers cost schools between pound;115-195 a day.
Growth in the use of support staff for cover was cited by 18 per cent of respondents, and 11 per cent blamed the drop in work on the employment of internal cover teachers by schools.
Other reasons included less teacher absence (4 per cent) and fewer training courses during the school day (5 per cent).
The NUT found that three-quarters of supply teachers had taught for more than 11 years and believe the education system is in danger of losing a wealth of experience.
Pru Nixon found supply work easy to come by during the 10 years she lived in east London. But since moving to Hatfield Heath near Bishop's Stortford, Hertfordshire, 18 months ago, the infant teacher has found that it has been increasingly difficult to find jobs.
Initially she had to drive no more than six to eight miles to work, but now often finds her nearest offer is 30 miles away. "I think a lot of it is down to school budget problems," she said.
But Tom Hadley, a spokesman for the Recruitment and Employment Confederation, representing some of the largest agencies, said his members found demand for staff had remained fairly constant. He said many provided a high level of continued professional development.
The NUT is calling for more local authorities or federations of authorities to re-establish supply teacher pools, along the lines of the Go Teaching agency set up in the South-west.
It also wants the Government and councils to invest in training for supply teachers and to ensure they are subject to the same terms and conditions as other teachers.
Steve Sinnott, NUT general secretary, said: "I am determined that the unappreciated and neglected situation in which supply teachers find themselves changes for the better."
The NUT found that 31 per cent of supply teachers had not been offered professional development within the past three years or more and 33 per cent had not been offered any by their supply agency.
The most common problem cited was pupil behaviour and attitude - 79 per cent of respondents - followed by insufficient planning left for them - 76 per cent - inadequate resources - 52 per cent - and lack of support from other staff - 41 per cent.
The NUT sent questionnaires to a random but geographically representative sample of 1,000 of its supply teacher members in England and Wales and received 230 responses.