The RMT and Aslef unions said this morning they will ballot Night Tube drivers for industrial action over a "block on career progression".
Both unions are protesting what they see as obstruction of drivers' career progression. Aslef has set a deadline of 10am on Thursday 16 March for the return of ballot papers.
According to the RMT, Night Tube train operators "have been prevented from moving into vacant full-time positions for a period of at least 18 months". The union said all other staff, including part-time Night Tube station staff, are eligible to apply but Night Tube train operators are not - which the RMT described as "outrageous".
The union also noted London Underground "has advised that Night Tube train operators do not attract the overtime rate of pay until they exceed 35 hours in a week".
As Night Tube train operators are currently contracted to only 16 hours, they are prevented from being compensated for any late finish due to, for example, service disruption.
The RMT said both these issues had led to the decision to ballot members for strike action and action short of a strike.
The news comes as commuters prepare for another strike on the Tube, which will begin tomorrow evening.
"It is outrageous that London Underground have decided to discriminate against their Night Tube drivers," said RMT general secretary Mick Cash.
"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."