The NUT is considering balloting its members over industrial action against increased pension contributions from teachers, The TES has learned.
The move comes after the ATL - one of the most moderate unions - announced this week that it was already preparing a vote over action against government plans that would see pension contributions increased by about 50 per cent.
NUT general secretary Christine Blower and ATL general secretary Mary Bousted were expected to be among 55 public sector union leaders gathering to discuss co-ordinated action over pension changes and government cuts at the TUC today.
But it is thought unlikely that the NUT will confirm a ballot until a clear cross-union plan of action is decided. Its last national strike action over pay in 2008, which it undertook alone, was supported by only a quarter of its membership.
Ms Blower said: "Like the ATL, we consider this a very serious matter and the NUT executive has already discussed balloting members. We will be discussing this matter further with the ATL and other teaching and public sector unions.
"All of the UK teacher organisations are united in opposing unnecessary pension cuts which will make our members poorer in retirement."
Chris Keates, general secretary of the NASUWT, said: "Anger will continue to ferment and deepen unless the coalition Government responds positively and swiftly now to the TUC's repeated representations to engage in meaningful discussions on public sector pay and pensions."
Unions are furious that the Government wants to settle the detail of how to implement the changes to teacher pensions by next month, a month before the independent Hutton report into public sector pensions is published. The proposals would see a rise of about #163;100 a month for the average teacher by 201415.
They have also expressed concerns that an up-to-date valuation of the Teachers' Pension Scheme has not yet been carried out.
The ATL, which has about 122,000 members, is traditionally viewed as a moderate union that shies away from strike action.
But deputy general secretary Martin Johnson said: "Our consultation with members shows the anger is deep and the majority (of members) want some action taken."