Drivers on Southern rail are to be balloted on industrial action in the ongoing row over driver-only trains.
The RMT union said its members would be asked to vote on both strike action and action short of a strike.
It has been embroiled in a bitter dispute with Southern over changes to the role of conductors.
Southern – which said the ballot move was “unnecessary” – wants drivers, rather than conductors, to operate carriage doors at certain times.
The RMT wants to retain the role of conductor, in addition to the driver, on trains at all times, citing safety.
The main drivers’ union, Aslef, is also balloting its members in the same dispute, raising the possibility of joint strikes in December.