Schools across south-east Wales are being misadvised over new pay arrangements, leading to the "sheer exploitation" of classroom teachers, according to a teaching union.
The NASUWT Cymru has declared a collective dispute on behalf of primary members in Monmouthshire, who it says are being asked to take on more responsibilities without extra pay.
But headteachers insist leading a curriculum subject area is a normal part of a primary teacher's job, and Monmouthshire local education authority is standing by similar advice.
The row is the biggest so far in Wales over the replacement of management allowances by new teaching and learning responsibility (TLR) payments.
Welsh heads have until March 31 to come up with new staffing structures for their schools which incorporate the new payments - ranging from pound;6,500 to pound;11,000 for TLR1s and pound;2,250-5,500 for TLR2s.
To be eligible, teachers have to have a significant role in improving teaching and learning across the school and, for the higher TLR1s, to manage a significant number of staff.
The dispute could affect schools across south-east Wales because the union is challenging advice given to Monmouthshire and neighbouring LEAs by the South East Wales Consortium.
The consortium, set up by Monmouthshire, Torfaen, Blaenau Gwent, Cardiff and Newport, advises on workload reforms and staff restructuring. Andrew Keep, Monmouthshire's director of education before his appointment last August, led the consortium.
NASUWT Cymru says primary teachers in Monmouthshire and other schools in the region are being asked to take on extra responsibilities - for example, leading school work in particular subject areas - without a TLR.
It says it has written to all five education directors in the region requesting a meeting to resolve the problem, but has been rebuffed.
Tim Cox, NASUWT national executive member said: "The new pay arrangements were designed to be fair to all teachers. What is happening in Monmouthshire is just sheer exploitation. Teachers are being expected to take on onerous and significant roles where they can be held accountable to Estyn inspection teams and heads but are not being paid for their additional responsibilities."
But Pat Clarke, immediate past president of the National Association of Head Teachers Cymru, said leading on one or more subjects was a routine part of a teacher's role in primary schools.
And Malcolm Morris, Monmouthshire's head of school improvement, said it was standing by the advice being given to heads by the Consortium.
"They are the people who have the knowledge and deal with national organisations, including the employers and the rewards and incentives group (which proposed TLRs)," he said.
"NASUWT is disputing the advice because some of its members appear not to have done so well out of this as others.
"It is only recommendation and advice, and heads and governors must decide how to play it."
The Assembly government declined to comment on the dispute in Monmouthshire. Pay and conditions are not devolved to the National Assembly. Elsewhere, NASUWT has held strike ballots over TLRs in Merseyside, Derbyshire, Lancashire, Manchester and Caerphilly.
Its rival, the National Union of Teachers, is holding three indicative ballots in secondary schools in Wales, and has held strikes at 15 schools in England and more than 30 ballots.