Heads 'angry over pensions' betrayal'
Head teachers say they are likely to vote for a ballot on whether to strike over plans to change their pensions, at their annual conference in Brighton.
The National Association of Head Teachers, which has members in England, Wales and Northern Ireland, says heads "feel betrayed" by plans to curb public sector pensions.
Heads could lose an average of £100,000 from their overall pensions, it says.
Ministers are urging unions not to rush decisions on industrial action.
The government is in talks with the unions and says it aims to announce firm proposals for all public sector pensions in the autumn.
From next April, teachers and other public sector workers are being asked to pay higher contributions to their pensions.
The Hutton review into public sector pensions recommended most employees should switch from "final salary" pension schemes to those based on a "career average".
They should also retire at 65, in line with changes to the state pension, he said.
Heads and teachers say this will mean they will work longer - for less money.
Russell Hobby, the general secretary of the National Association of Head Teachers (NAHT) says heads feel angry and betrayed.
Many see the pension scheme as a reward for their accepting lower pay than than they might have earned in the private sector, he said.
"They rightly see the proposals as an attack on the fair rewards from a lifetime of public service, " he said.
"We are seeing a loss of about £100,000 from an average head teachers' pension. That feels like a real betrayal to the profession."
The union will vote on Sunday on whether to hold a ballot for industrial action up to and including a strike. Mr Hobby says a "yes" vote is "very likely".
If the motion is passed by the conference, it is likely the union will ballot - and take any action - in the autumn.
But schools could be affected by strikes over pensions before then. Two classroom teachers' unions voted for similar action at their annual conferences recently and they may ballot this term.
The NAHT is strongest in primary schools - where it says it represents 85% of heads in England, Wales and Northern Ireland.
It says it represents 40% of secondary heads.
The government is in talks with the unions and says it will announce firm proposals for all public sector pensions in the autumn.
It aims to set out "general principles" for changes to teachers' pensions in England and Wales by the end of June. Teachers' pensions in Scotland and Northern Ireland are decided by the national administrations.
Schools minister Nick Gibb recently told a teachers' conference that public service pensions should remain a gold standard - but that rising costs and greater life expectancy meant reform was needed. The government is urging teachers - and head teachers - not to rush their decisions.