Just months after finally hitting the tracks, Night Tube services could be halted by industrial action.
Drivers who are part of the RMT (Rail, Maritime and Transport) union are balloting for a strike over a dispute about working conditions and overtime pay.
The result will be announced on 14 March, meaning strike action would be likely to take place towards the end of the month.
According to the Evening Standard, RMT leader Mick Cash said that drivers on the Night Tube, which currently operates on five London Underground line, are being “blatantly” discriminated against because of rules on overtime pay and promotion.
He said: “It is outrageous that London Underground (LU) 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 would call 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.”
The Night Tube currently operates on the Central, Victoria, Jubilee, Piccadilly and Northern lines.
The news comes as Londoners brace for a strike on the Central and Waterloo & City lines, beginning at 11.30pm on Tuesday evening.
The 24-hour strike involves members of the RMT and Aslef unions.
There will be no service on the Central line east of Leytonstone and services on the rest of the line will be reduced.
There will be no service at all on the Waterloo & City line.