Drives on the London Night Tube are to be balloted over potential strike action, the RMT union has confirmed.
The decision is the latest stage in a long-running dispute over pay and jobs on the newly launched service.
Night Tube services launched last August after a series of delays caused be the dispute between TFL and the RMT over pay and conditions.
The RMT claims the latest action has been caused by TFL’s refusal to allow drivers to move into vacant full-time jobs.
The Night Tube currently runs on Friday and Saturday nights on the Central, Northern, Victoria, Jubilee and Piccadilly lines.
RMT general secretary, Mick Cash, said: “It is outrageous that London Underground have decided to discriminate against their Night Tube drivers.
“This is a senseless and damaging policy that picks out one group of staff for negative treatment and of course the drivers are angry and that is why we are balloting for action.
“RMT calls on London Underground to see sense, stop these stupid attacks on career progression and fair reward for overtime working and confirm that this discrimination against the Night Tube drivers has been lifted.”
The union said Night Tube drivers were only eligible for overtime pay as they were contracted to work only 16 hours a week.