Qualified supply teachers are being forced to accept work as teaching assistants and cover supervisors at much lower rates of pay, it has emerged.
Many supply teachers have complained that their normal supply work has "dried up" because schools are taking the cheaper option of employing cover supervisors.
Some qualified teachers have turned down cover supervision roles out of principle, but others have been forced to take it in order to pay their bills.
The advent of "rarely cover" in September has increased the need for extra staff to cover for absent teachers, because their colleagues can no longer do it during free periods.
But supply teachers say heads are freezing them out and replacing them with agency and in-house cover supervisors, in order to save money on their increased wage bill.
Preliminary findings from an investigation by The TES show schools in some areas employ teams of up to 10 cover supervisors.
One woman responding to a new survey of 450 supply teachers by the Association of Teachers and Lecturers said she had accepted a permanent post as a cover supervisor that was paid just pound;1 more per hour than the school cleaner.
Another teacher, writing on The TES forums, said they took cover supervisor work because it was better than staying at home and earning nothing, but added: "If I go in as a cover supervisor, they get a cover supervisor. I give the work out and do my best to ensure they do it."
The Recruitment and Employment Confederation, which represents supply teaching agencies, blamed the schools for increased demand for cover supervisors.
But it said teachers are only offered cover supervisor roles if they have previously expressed a desire to take the work, or the agency felt the work involved in that role amounted to "teaching".
John Dunn, head of the confederation's education group, said the use of cover supervisors to replace supply staff was a "big scandal waiting to blow up", and was leading to an increasingly unregulated workforce.
Tony Callaghan, a former headteacher who recently took a job as a cover supervisor in a Norfolk secondary to expose what the job entailed, said the tast was "impossible" without teaching to some extent.
According to guidelines, cover supervisors are not allowed to actively "teach" and should only be used for short absences.
Campaigners say the issue of poorly paid cover supervisor roles hitting demand for qualified teachers is just one of many concerns. They are already worried about how supply teachers will be affected by the new "licence to practice", since most have very poor access to continued professional development.
Unions have also made repeated calls for agency supply teachers to be included in the Teachers' Pension Scheme.
Lucy Orchard, a 28-year-old supply teacher in Devon and Somerset, is organising a protest march in central London on April 9. She said: "The reason we struggle to be heard is that they don't talk to each other enough."
Martin Ward, deputy general secretary of the Association of School and College Leaders, said: "I can see why a supply teacher might be worried when there is less work coming their way that it might be because of cover supervisors.
"However, the `rarely cover' rules mean there is actually less cover being done, with headteachers reducing the number of trips and visits and other optional occasions."
THE TEACHERS WAITING IN THE WINGS
- In March 2009, there were 50,999 people registered as supply teachers with the General Teaching Council of England.
- One in 10 registered teachers is a supply teacher.
- Supply teachers can be employed by independent agencies, local authority-run agencies or directly by the school.
- Agencies can charge schools up to pound;200 per day for a qualified supply teacher, although the teacher might receive significantly less than this.