Primary classroom teachers who were hoping to get extra pay for being subject leaders under the new salary system are likely to be disappointed.
The reforms of management allowances, expected to be introduced in Wales from next January, are intended to reward responsibility for teaching and learning.
But Welsh headteachers have been advised that if all their teachers are subject co-ordinators, none could be considered to have the significant or additional responsibilities required to qualify them for new "teaching and learning responsibility" (TLR) points.
As more than two-thirds of Welsh primaries have nine or fewer teachers, all of whom would usually be responsible for at least one subject, the ruling would affect most staff.
The advice, given by a local authority workload adviser at last month's education exhibition in Cardiff, has shocked heads and some teacher unions.
Kevin McAnulty, head of Abercarn primary school, Caerphilly, said:
"I'm surprised and I think it will cause us serious problems. One of my co-ordinators is responsible for science, IT and music. I thought she would get a TLR."
The current system of management allowances, worth between pound;1,638 and pound;10,572, will be replaced from next January by TLR1 and TLR2 payments of pound;6,500-pound;11,000 and pound;2,250-pound;5,500 respectively.
John Hogan, a remodelling adviser with Caerphilly council, said both sets of payments were intended for people leading improvements in teaching and learning across the school extra to those borne by ordinary classroom teachers - for example, developing a curriculum area or enhancing good practice. Only those who also manage a "significant" number of other staff should receive the higher TLR1.
Speaking at the education exhibition, he added: "Primary subject co-ordinators by themselves don't qualify if everyone is a subject co-ordinator, because it's not additional. If everybody co-ordinates subjects, that's the basic job of the teacher in the school - so you can't argue it's additional.
"It's got to affect other children, not just your own class, and involve pupil development across the curriculum. It's got to be more than an ordinary responsibility."
The Assembly government is consulting on draft regulations which would require heads to produce a revised staff structure, setting out which posts would attract TLRs, by December 31.
Similar regulations were implemented in England last month, and the Department for Education and Skills has already issued guidance.
The National Union of Teachers Cymru has called for schools in Wales to be given an extra term - to Easter 2006 - to carry out their staff reviews.
Assembly guidance on how to go about the staff review is not expected before the end of this month or early July.
Gethin Lewis, NUT Cymru's secretary, said: "We must not rush it. The schools that are going to be most affected are the small ones."
Mr Hogan urged heads to start work now, with a view to delivering a draft staff structure to governors before the end of this term.
But Anna Brychan, director of the National Association of Head Teachers Cymru, expects most schools to wait for the regulations to come into force, meaning work will not start until September.
Many NAHT members support extending the January deadline for completing staffing reviews, but this could lead to confusion for teachers about safeguarding current allowances, she said.