Drivers on London's Night Tube are to be balloted for strikes in a row over pay and jobs.
Members of the Rail, Maritime and Transport union will vote in the coming weeks on whether to launch a campaign of industrial action.
The union claimed drivers on the weekend service, which started last August, had been "blocked" from applying for full-time jobs on London Underground.
The RMT said it was a "blatantly discriminatory" policy which 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."
Peter McNaught, London Underground operations director, said: "Night Tube services have been running since August last year in line with agreements reached with the unions.
"We invite the RMT to meet with us to discuss any issues that need to be resolved and there is no need to threaten industrial action."