The two-year pay settlement for teachers in England and Wales could be reviewed because of rising inflation.
Two teachers' conferences have already voted for pay strike ballots
The independent body which advises the government has asked for its permission to reconsider the current pay deal. Ministers are considering the request.
The settlement, with 2.5% rises last September and next, included scope for a review if inflation were to pass 3.25%. The headline rate is now 4.8%.
The NUT teachers' union said this meant they faced "a significant pay cut".
It is understood that if the School Teachers Review Body is given the go-ahead it will start the process from scratch, taking fresh evidence and covering both school years from 2006 to 2008.
WHAT TEACHERS EARN
newly qualified, after four years at university: £19,641
after seven years: £31,098
head teacher: 38,559 - £95,631 depending on school size
figures exclude London and fringe area weightings
NUT General Secretary Steve Sinnott said: "Teachers have faced boom and bust in their pay levels for more than 30 years.
"They are given a boost only to see it eroded by inadequate settlements and inflation. We are determined that will not happen again."
He said it had been clear for some months that the current "imposed settlement" - the NUT prefers collective bargaining - was inadequate.
"Schools have faced teacher shortages in the all-too-recent past and they do not want to go back to a position where they are scouring the world to fill the gaps," Mr Sinnott said.
The government has set a public sector pay target of just 2% - prompting moves among some unions to ballot for strike action.
It believes inflation will be back to that level by the end of the year.