The National Association of Schoolmasters Union of Women Teachers and the Association of Teachers and Lecturers have complained to the Trades Union Congress about the National Union of Teachers trying to poach members.
And there was further evidence of soured relations this week when the NASUWT did not take part in an NUT-initiated bid to present a united front on pupil assessment.
The joint statement, from the Professional Association of Teachers as well as the ATL and NUT, renewed a call for national tests and league tables to be scrapped and advocated an alternative system based on teacher assessments.
Material sent to NUT members deepened its rift with other unions over the workload agreement, by dubbing last month's deal on performance pay for the upper scale as "The Betrayal".
The NASUWT and ATL also claim the NUT is attempting to poach members with a poster advertising reduced subscriptions for those joining from other unions and are furious at its description of them as "the Government's unions".
In a letter to members, Eamonn O'Kane, NASUWT general secretary, says the "persistent, unfair and corrosive attacks" from the NUT have undermined any possibility of co-operation with it.
An accompanying handout entitled "Has the NUT gone nuts?" described it as a "paper tiger" whose leadership was gaining the "opprobrium and contempt" of the trade union movement.
Doug McAvoy, general secretary of the NUT, which continues its controversial advertising campaign in The TES this week accusing other unions of being prepared to increase class sizes, said the NASUWT's response was "pure invective".