The NASUWT backed an emergency motion at its annual conference in Birmingham last week, expressing fear that the Assembly government was taking steps which would lead to Welsh teachers being paid differently to those in England.
Chris Keates, general secretary, said such a change would inevitably cause pay cuts as Wales is a lower-paying economy with fewer school staff shortages.
She said the Assembly government and the General Teaching Council for Wales (GTCW) had been developing separate professional standards for teachers, which indicated a possible move to a separate pay structure.
The GTCW has developed a framework that would ensure experienced teachers who do extra training get professional recognition of their skills. But it would be independent of pay.
Ms Keates said: "We wouldn't mind a separate pay system if it was for the better, but experience suggests that wouldn't happen."
Neil Butler, Powys NASUWT negotiating secretary, said a separate pay system was "an obvious temptation for the Welsh Assembly, but one of our biggest fears".
Delegates backed the motion to "resist, by all appropriate means, any attempt to devolve fully to the Welsh Assembly responsibility for teachers'
pay and conditions".
But Jane Davidson, education, lifelong learning and skills minister, said:
"We have no intention of bringing pay and conditions to the Assembly while there is no appetite for it in advance of the primary law-making powers being transferred to the National Assembly."