London's Night Tube is facing strike action as the RMT ballots drivers in a dispute over career progression, the union announced on Monday (20 February).
The RMT has accused Transport for London (TfL) of being "blatantly discriminatory" against the part-time operators, who work on Friday and Saturday nights on major Underground lines.
The union has alleged that the drivers have been prevented from moving into vacant full-time positions for a period of at least 18 months, while other staff, including part-time Night Tube station staff, are eligible to apply for the roles.
"It is outrageous that London Underground have decided to discriminate against their Night Tube drivers," said Mick Cash, the general secretary of the RMT.
"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 would call on LU [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."
Peter McNaught, LU 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."
The strike ballot closes on Tuesday 14 March, but the news comes just a day before Underground drivers plan to walkout for 24-hours from 21 February on the Central and Waterloo & City lines.
The action would mean no Central line service east of Leytonstone and a reduced service on the rest of the line and no service at all on the Waterloo & City line, according to TfL.
McNaught added: "I apologise to customers for the disruption that this unnecessary strike will cause.
"We have made all reasonable efforts to resolve this dispute through talking through the issues with the unions, and we have minimised the number of employees affected from over 30 to eight.
"All of these moves are within the long-standing agreements we have made with the unions. We urge them to engage with us to resolve this issue and to withdraw the strike action, which will only cause needless disruption to Londoners."