Supply teachers could see their pay hit hardest by the introduction of performance-related pay in schools, the classroom unions have warned.
Teachers who leave the profession could also find themselves earning a lower salary if they return to the classroom.
As a result of the abolition of the spine points on the main pay scale under the government's controversial reforms, teachers will lose out on so-called "pay portability", which ensured they maintained their salary level when moving schools.
Simon Stokes, ATL’s policy adviser on pay and conditions, told TES that supply teachers who are hired directly by schools could be particularly disadvantaged by their pay now being set at the discretion of their school.
"I hope schools will look at the experience teachers bring and that this will have a bearing on pay, but I think it will be unlikely," he said.
Chris Keates, general secretary of the NASUWT, said that many supply teachers hired through agencies "are already being exploited by supply agencies and are being paid at lower levels than their experience merits".
The new pay structures mean that supply teachers directly hired by schools could also lose out, she added.
While the pay of teachers on the upper pay scale who remain in the profession is protected by the School Teacher’s Pay and Conditions Document, those who have a "break in the continuity of their employment", such as returning from retirement or taking time off work after their maternity leave has ended, will see their salaries set at the discretion of the head.
According to official figures, around 10,000 teachers return to the profession each year.
Mr Stokes warned that as schools are no longer legally required to match new teachers' previous salaries, many could well receive significantly lower rates of pay in the future.
"Schools will always try to save money and try to get the best deal from the teacher," he added.
Sally Langran, specialist pay adviser for the NAHT heads' union, said that while she expected many teachers would be paid at their prior rate, there would be a "mixed bag" for others.
Reporting by Will Sillitoe