Drivers on London's Night Tube are to vote on strike action in a long-running row over pay and jobs.
The Rail, Maritime and Transport union said it was balloting members in the coming weeks on whether to launch a campaign of industrial action.
The RMT claims drivers on the weekend service, which started last August, had been "blocked" from applying for full-time jobs on London Underground.
Calling it a "blatantly discriminatory" policy the union says it prevented Night Tube drivers from moving into vacant full-time positions for a period of at least 18 months.
All other staff, including part-time Night Tube station staff are eligible to apply, it was claimed.
The drivers are not eligible for overtime pay because they are contracted to work only 16 hours a week, the union added.
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 LU 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."