The decision by teachers in the moderate ATL union to go on strike last week could inspire thousands of heads to take to the picket line, a heads' leader has claimed.
The Association of School and College Leaders' (ASCL) consultation with members on whether to hold a ballot on industrial action over proposed changes to their pensions comes to an end today.
General secretary Brian Lightman said it had yielded a "big response" from its 15,000 members, which include heads and other senior school and college leaders.
"That must be evidence of the strength of feeling," he said.
If ASCL decides to proceed with a ballot, this raises the prospect of members joining their colleagues in fellow heads' union the NAHT - which will ballot its members in September - and the classroom unions for further action in the autumn. This would happen only if the TUC-led talks with the Treasury fail to deliver an agreement.
ASCL deputy general secretary Martin Ward told The TES that even the union's more conservative members were "very cross" about the proposals, which would see their contributions increase by 50 per cent, the retirement age rise and the scheme switch from a final salary basis to a career average.
When asked whether ATL's decision to strike for the first time in its 127- year history could influence ASCL's members, he said: "I think so. ATL, like us, has a long history of not taking industrial action. Our members are very angry about it as well. They are public servants who feel themselves to be part of middle England, and feel let down by a government they might have expected to be supporting them."
In an ASCL survey earlier this year, 63 per cent of members said they were prepared to take action to protect their pensions.
Mr Ward added: "We know we have members at both ends of the spectrum, some who are demanding to know why we haven't done something already, and some who will leave as soon as we say we are going to take action.
"People are very cross. Whether that will translate into strike action I don't know. Certainly there are those who think we should have been on strike last week."
If the NASUWT - whose general secretary Chris Keates has already warned of the "inevitability" of a strike without a radical shift in the Government's position - joins forces with the NUT, ATL and two heads' unions, almost every school in the country would be disrupted.
The impact would dwarf the scale of last Thursday's strike, which caused more than 12,000 schools to fully or partially close.
About 85 per cent of state schools and colleges are believed to have been affected to some extent.
Education secretary Michael Gove described the action as "disappointing and unnecessary".
ATL general secretary Mary Bousted said that she was "proud" of her members for making a stand, while NUT general secretary Christine Blower said: "The response from all of our unions, the NUT, ATL, UCU and the PCS, has been fantastic."
STRIKE-DAY BANNERS `Mr Cameron. You've let us down, you've let the country down, but most importantly . you've let yourself down' `Will I still teach you, even if I can't see you, when I'm 67?' `Carmel Islington prays for you, Gove' `Thatcher stole my milk, Cameron is stealing my pension' `Write the following out 68 times . "I'm sorry for living longer"' `If you can read this, thank a teacher with a fair pension!' `Francis Maude is a girl's name'.
`Mr Cameron. You've let us down, you've let the country down, but most importantly . you've let yourself down'
`Will I still teach you, even if I can't see you, when I'm 67?'
`Carmel Islington prays for you, Gove'
`Thatcher stole my milk, Cameron is stealing my pension'
`Write the following out 68 times . "I'm sorry for living longer"'
`If you can read this, thank a teacher with a fair pension!'
`Francis Maude is a girl's name'.