Peace may have broken out between the factions of the National Union of Teachers, but Easter’s conferences signalled all-out war by teachers on heads who fail to give them time for marking and lesson preparation this September.

The National Association of Schoolmasters Union of Women Teachers, the union most loyal to the workforce agreement, threatened heads with legal action if they did not comply.

Two secondary schools, St Gregory’s RC in Harrow, London, and Barr Beacon in Walsall, have already faced one-day strikes, after refusing to bring in the deal, which cuts teachers’ working hours.

The NASUWT, which held its annual conference in Brighton this week, is urging members to report heads who break the law. Chris Keates, general secretary, accused heads of taking “unlawful, unballoted industrial action” and abusing their positions.

She said: “I am not sure that relationships can get any worse. Where heads continue to deny their staff their legal entitlements, relationships are already beyond repair.

“We have no intention of allowing a maverick group of heads to deny our members the benefits of workforce remodelling.”

Stephen Twigg, the school standards minister, told the NASUWT conference:

“From September PPA (planning, preparation and assessment) time is a statutory requirement. Any head who thinks that PPA is not for their school has got it wrong. Contractual obligations will be enforced.”

The National Association of Head Teachers voted, against its leadership’s wishes, to pull out of talks on the agreement, which frees teachers from more than 20 tasks and gives them 10 per cent of the working week for PPA, because of funding concerns.

A government spokesman said: “There are different legal options for seeking enforcement. The most obvious route is for a teacher to claim constructive dismissal (after having pursued the statutory grievance procedure) and bring a claim in the employment tribunal. A claim to the county court is another possibility.”

The heads says they cannot implement the agreement because they do not have enough funding, a claim Ms Keates described as “spurious”. She said no head had yet responded to offers of help from the Department for Education and Skills, which said it would assist those who could prove, by showing their books, that their budgets were stretched.

Joe Boone, head of the NASUWT’s industrial relations section, said: “We know that primary schools have pound;750 million in rainy-day savings.

They could have implemented workforce reforms a decade ago if they had wanted to. But the fact is that they do not want their staff to have PPA time.”

David Hart, general secretary of the National Association of Head Teachers, said: “I am not surprised that the NASUWT are taking this stance. We will be repeating to NAHT members our advice that this is a contract which has got to be delivered, unless there are exceptional circumstances.”

However the NAHT’s action garnered a very different reaction at the NUT’s conference in Gateshead.

A motion on workforce reform opposing “social partnership” (between unions and government), job cuts and modernisation, was backed by a large majority of NUT delegates. They also voted for strike ballots at schools that insisted on using support staff for teaching duties and a national ballot over a one-day strike to secure the funding needed to implement PPA time.

However Steve Sinnott, the NUT’s general secretary, played down the threats of strikes, indicating that he hoped to move foward by negotiation.

He said he would use a meeting with Ms Kelly next month to pursue his union’s case.