More Night Tube drivers have voted for strike action in a row over "fair access to full-time jobs" and will strike on the nights of Saturday 8 April and Saturday 29 April.
Members of train drivers' union Aslef have voted in favour of a strike and action short of a strike. As they represent the majority of the drivers, it will cause significant disruption to the Night Tube services if an agreement with London Underground isn't reached.
Aslef's organiser on London Underground, Finn Brennan, said the union was "determined to achieve a fair deal for Night Tube drivers just as we are for every other member".
Around 100 RMT drivers are also set to strike on the same dates after they voted for a walkout and action short of a strike earlier this week.
Both unions have been protesting what they see as obstruction of drivers' career progression. They say "a blatantly discriminatory" policy means Night Tube train operators have been prevented from moving into vacant full-time positions for a period of at least 18 months.
Brennan said Aslef has "repeatedly told" London Underground it was unfair "and potentially unlawful" but it hadn't listened to the union's concerns.
RMT general secretary Mick Cash said:
It is outrageous that London Underground have decided to discriminate against their Night Tube drivers.
Peter McNaught, operations director for London Underground, said:
Night Tube services have been running since August last year in line with agreements reached with the unions.
We’ve met with the unions to discuss and resolve the new issues that they have.
We have invited the unions to more talks this week and I encourage them to continue discussions with us rather than threaten industrial action.
To date, the Night Tube services, which are up and running on five lines, have been more popular than anticipated.
In November last year, London Underground's managing director Mark Wild said passenger numbers were 50 per cent higher than forecast.
The services were first launched on the Victoria and Central Lines on 19 August and then joined by the Jubilee on 7 October. The Night Tube then came to the Northern and Piccadilly Lines.