The NASUWT leadership will attempt to change theunion's rules to bar members of other unions from holding elected posts at a special conference later this month.
The move is widely regarded seen as a way of preventing Hank Roberts - a campaigner for a single-teacher union - from winning a place on the NASUWT national executive. It will be debated in Birmingham on April 29.
Mr Roberts, who is already a member of both the ATL and NUT executives, attacked the move as undemocratic. He said: "I am greatly saddened by this.
If members want to elect me, why should they be prevented?"