At the start of the teachers' Easter conference season, the general secretary of the Association of Teachers and Lecturers, Mary Bousted, described a small group of heads as "vociferous refuseniks" who were taking the moral low ground, and warned that the union would "take them on".
Relations between teachers and heads deteriorated last week when the National Association of Head Teachers (NAHT) said it was pulling out of the government's deal designed to lighten teachers' workload.
Speaking at the ATL's annual conference in Torquay yesterday, Dr Bousted said: "This is the one thing they have been asked to do which will give teachers and schools a reasonable work-life balance and stop them suffering from stress and ill-health through overwork."
The union would back its members "up to and including strike action".
Dr Bousted said part of the problem was the attitude of some male heads.
"Quite a few male headteachers at primary schools like being cock of the roost and don't like the thought that their hens can go out to get professional development in their own time," she said.
The warning from the ATL, the third largest teaching union, follows a similar threat from the National Association of Schoolmasters Union of Women Teachers last week.
Headteachers voted narrowly not to implement the reforms, due to be introduced in September, because their union says the government has not provided enough money to make them work.
The general secretary of the NAHT, David Hart, last night described Dr Bousted's comments as "quite mind boggling".
"To allege that this is some form of gender issue is complete and utter nonsense," he said. "There is not a shred of evidence to support it."
He agreed that schools were likely to be disrupted by a "winter of discontent" as teachers prepared to strike over the issue.
The government has warned heads that they are legally required to implement the third phase of the workload agreement when it comes into force in September, guaranteeing teachers 10% non-contact time a week.
The threat of strike action is due to be debated during an emergency motion at the ALT conference today.
Dr Bousted's criticisms of the headteachers are echoed today in Education Guardian by Chris Keates, leader of the second largest teaching union, the NASUWT. "I don't think their stance on the workload agreement is helpful to us at all," Ms Keates says. "There's a group of headteachers - by no means all - who now think they are above the law, and that they know what's best."
Dr Bousted, Ms Keates and Steve Sinnott, leader of the National Union of Teachers, were taking part in their first joint interview.
