Headteachers are set to join an unprecedented wave of strikes this year if the Government does not back down on its plans to overhaul teachers' pensions, The TES has learnt.
Heads' union the NAHT is due to hear an emergency motion on the issue at its annual conference this weekend, following calls for a series of walk- outs by classroom unions.
The NAHT conference, to be held in Brighton, is expected to hear calls for the union to take "any action necessary" to fight against pension changes, including strikes.
The move by the NAHT follows anger over the Government's pension plans at the classroom union conferences held over the Easter holidays.
The NUT and the ATL have both confirmed that they will hold ballots of members in May and, if successful, the first strike will hit in June. Further action is expected in the autumn unless they win concessions from the Government.
NASUWT general secretary Chris Keates said her union would wait until talks with ministers had finished before deciding on industrial action, but it already has a mandate to ballot members if talks fail.
All of the unions are angry at proposals made by Lord Hutton - and welcomed by the Government - to axe final salary schemes, raise contributions by 50 per cent and gradually raise the retirement age.
A consultative poll of NAHT members in February indicated that any ballot for industrial action would be successful: 64 per cent of the 3,300 participants said they would take some form of action to protect their final salary scheme.
NAHT general secretary Russell Hobby said: "We are not involved in the TUC negotiations, but we have made our views known to the Government quite strongly.
"This issue is something members feel very strongly about. Heads have an appetite for a fight on this one and the evidence is strong from our surveys."
Mr Hobby said the association would be unlikely to arrange a ballot in time to co-ordinate action with other unions in June. Autumn is expected to be a more likely timeframe.
The 15,000-strong Association of School and College Leaders, which has never taken industrial action, is yet to make a clear statement on its position. However, a survey in March found that 63 per cent of members would take action to retain their current final salary scheme. More than two-fifths of those said they would be willing to strike.
The NUT and ATL are expected to co-ordinate strikes, with a successful ballot likely to close thousands of schools.
Jerry Glazier, of the NUT executive, said the strike threat sent a "clear and unequivocal message to the Government - we will not stand by and let them mess with our pensions".
The union also called for a general strike in the autumn with other public sector unions to protect pensions.
Paul McGarr, from east London, said that unless action in the summer term forced the Government to back down, it must be "the beginning and not the end of the battle".
John Blake, from Camden in north London, told the conference that the term general strike had "haunted the conference like a historically and politically illiterate poltergeist" - but the motion was carried almost unanimously.
Ms Keates said the NASUWT would exhaust negotiations with the Government in May before moving to industrial action.
Christina McAnea, head of education at support-staff union Unison, said it would ballot members "at some time this year" but said it wanted to ensure a high turnout beforehand.
Schools minister Nick Gibb told NASUWT members that he recognised the pension reform proposals had created "huge anxieties", but insisted discussions between ministers and the TUC had been "very constructive".
Shadow education secretary Andy Burnham, who also attended the NASUWT conference, said he would be "supportive" towards teachers who took industrial action, but ruled out appearing at picket lines.
"I can understand people are angry, and I understand their lack of trust," he said. "But I remember the disputes in the 1980s and I know the damage strikes can do."