Kay Driver, the "preferred candidate" of the Professional Association of Teachers for its general secretary, has been challenged for the post and a ballot will take place next month.
Ms Driver, deputy general secretary of the Secondary Heads Association, was chosen from a shortlist of six by the union's council.
Under trade union law a ballot of the membership is required if there is competition for the job. Dick Cribb, a PAT council member and field officer in Kent, and Ian Elfick, headteacher of a special school in Edinburgh and former chair of the union's Scottish Executive Committee, have now thrown their hats into the ring. The result will be announced on March 25.