Delegates to the National Association of Schoolmasters Union of Women Teachers conference threatened strike action against heads who attempted to "frustrate" implementation of the workload deal.
Sue Rogers, union treasurer, said: "If they try to block it for any reason we will fight them.
"They have got to keep to this agreement because it is in legislation. For the first time ever it is there in our contracts."
Eamonn O'Kane, NASUWT general secretary, attacked Phil Willis, the Liberal Democrat spokesman, for supporting the NUT's anti-agreement stance.
"You really shouldn't parrot NUT propaganda," he told Mr Willis, who addressed delegates at the Bournemouth conference."It is an outrageous claim that the agreement will result in double- sized classes.
"Does anyone seriously imagine five teacher unions and three others would sign up to such an agreement?"
And the rift between the unions seemed to put paid to the NASUWT exploring a teachers' super-union as delegates voted 89,273 to 65,295 against discussing a merger with other unions.
Proposals that the union should explore the possibility of a merger or agree to recruit only in certain areas of the education service were thrown out.
The NASUWT is now to explore forming a federation of TUC teacher unions and sharing the costs of some services. Mr O'Kane, who is known to favour a merger, said: "The environment the discussion took place in was not very helpful to even the most enthusiastic proponent of merger.
"The problems that have arisen between the NUT and, not just us, but the other signatories of the agreement resulted in an atmosphere which was quite inimicable."
A report on the future organisation of the NASUWT will now go out to consultation and will be discussed at next year's conference or at a special meeting.