Mary Bousted, Association of Teachers and Lecturers' general secretary, told her union's conference in Torquay that the National Association of Head Teachers would have to deliver the deal.
"You will not be left alone to go your own way," she was expected to say in her end-of-conference speech, a draft of which was seen by The TES. "We will be using all available means, including the law, to ensure the agreement is implemented."
An ATL emergency motion, regretting the decision by the NAHT extraordinary general meeting last week to pull out of the deal, was passed by an overwhelming majority. Heads are worried that there is not enough money to give primary teachers time for planning, preparation and assessment (PPA).
ATL delegates also voted to break with more than a century of tradition and admit heads to the union for the first time. Members suggested it might poach pro-deal heads from the NAHT.
Andy Ballard, Somerset executive member, said he looked forward to "hundreds if not thousands" of NAHT members transferring to the ATL because that was where they would get support in implementing the agreement.
He accused the NAHT of "grizzling" for three years, then "throwing its rattle out of the pram".
Mrs Bousted began the week by saying that the ATL was prepared to strike if heads refused to guarantee PPA time - and accused some of bullying staff over the deal. She said it was also a gender issue as "cock of the roost" male primary heads did not want to give their "hens" more freedom.
David Hart, NAHT general secretary, said her comments were "nonsense" and "mind-boggling". "If the ATL thinks it is going to be able to mount a recruitment campaign on the back of the workforce agreement, it has another think coming."
Meanwhile, the man favoured by NAHT's personnel committee to replace Mr Hart, who retires in the summer, still wants the top job. David Hawker had said he might pull out of the leadership election if the association withdrew from the workforce deal. But the head of children's services in Brighton believes he can still represent members, as they are not opposed to the principle of the deal but the lack of funding for it.