The ballot for general secretary of the civil service union PCS will open on 7 November and close on 12 December.
For the first time in 18 years, the sitting general secretary, Mark Serwotka, faces a challenge from the left.
Bev Laidlaw, the Independent Left candidate, got 17 branch nominations, topping the number of 15 required to get on the ballot paper.
Serwotka got 62 nominations. The candidate backed by the Socialist Party, Marion Lloyd, got 39.
The SP was a dominant force in the union, closely allied with Serwotka, until about a year and a half ago.
In the Assistant General Secretary