Urgent action is needed to stop supply agencies "undermining" teachers' pay and conditions and "disrespecting" the profession, a union has warned.
Members of Welsh-medium union UCAC are proposing three motions at their annual conference in Llandrindod Wells today designed to curb the growth of teaching agencies, which they claim are creating "genuine hardship" for teachers.
The union wants the Westminster government to close legal loopholes that allow agencies to set their own pay and conditions - which the union claims can be as low as pound;50 a day. It is calling on headteachers to stop using supply agencies and for local authorities to keep their own supply rosters.
Rebecca Williams, UCAC's policy officer, said: "A teacher working through an agency does not receive the same salary or have the same working conditions as other teachers.
"The amount the school pays the agency is already less than the statutory pay, and the agency takes a further cut from that before paying the teacher. So many agencies are competing for business, particularly in South Wales, that they are now severely undercutting each other.
"This is a very detrimental situation for what is supposed to be a profession with statutory pay and conditions."
UCAC said it had anecdotal evidence of pay as low as pound;50 a day for supply teachers, although it can be up to pound;120. Few supply agencies offer extra pay for experienced teachers. By comparison, a newly-qualified teacher can expect to earn pound;21,588 a year, increasing to pound;31,552 after six years.
Because they are not bound by statute, few supply agencies offer their teachers a holiday allowance, planning and preparation time, sick pay or pension contributions.
One UCAC member, a teacher with 35 years' experience who did not want to be named, said he signed up with two agencies before he retired recently. He claimed that when one agency found him work, the other contacted the school and offered his services at a lower price.
There are currently 5,200 registered supply teachers and 37 teacher supply agencies in Wales.
One of the largest is Cardiff-based New Directions Education, which has eight offices in Wales and England and 3,000 supply teachers on its books from Wales alone.
The company was launched in 1999 and turned over pound;10 million in the last financial year.
Managing director Gary Williams said: "Agencies like ours are an integral part of the education system. We provide a cost-effective and convenient service for schools and councils, saving them time and money. If we were abolished I don't think the system could cope."
New Directions has preferred supplier agreements with a number of south Wales local authorities. It takes a commission of around 17.5 per cent and pays teachers from pound;90 a day.
Mr Williams said he would welcome discussions with teaching unions about the role of agencies.
Gareth Jones, secretary of ASCL Cymru, said supply agencies have their place in the system but agreed it was not an "easy situation" for school leaders.
"Heads and governing bodies have decisions to make about how they deal with temporary staffing situations, and there's sometimes a trade-off between the funds available and the needs of the school," he said.
NASUWT Wales organiser Rex Phillips said: "It's regrettable that far too many heads and governing bodies are too eager to bring in teachers from supply agencies just because they provide them more cheaply."
A Department for Education spokesman said the Government was committed to "freeing up" teachers' terms and conditions to allow heads more freedom to reward staff "as they see fit".
"Pay for supply teachers will be part of the broader considerations given to ways in which reforms to the pay system could guarantee schools greater freedoms to meet their local needs and priorities," he said. "We will set out next steps shortly."