Drivers on London's Night Tube are to stage two strikes on Saturdays next month after voting in favour of action in a dispute over jobs.
The Aslef union said all its members backed industrial action in a turnout of 75 per cent, adding that there was "ample opportunity" for talks to avert the planned walkouts on April 8 and 29.
The Rail, Maritime and Transport (RMT) union has already revealed that its members voted by more than 9-1 to strike.
The unions say drivers on the weekend service, which started last August, are being blocked from applying for full-time jobs on the Tube and do not qualify for overtime pay.
Aslef official Finn Brennan said: "Under the previous mayor, London Underground pushed through the recruitment of part-time staff to operate Night Tube because they weren't prepared to find a fair way to roster the work with full-time staff.
"Boris Johnson believed that new part-time recruits could be used to undermine trade unions and attack conditions for existing staff.
"Instead, these new recruits saw that it made sense to join and get active in Aslef and have now delivered a huge Yes vote to strike.
"Aslef is determined to achieve a fair deal for Night Tube drivers just as we are for every other member.
The Union has given TfL more than a month to prevent the strikes.
Mr Brennan added: "I hope they will take advantage of this opportunity to avoid the necessity to take strike action."
London Underground operations director Peter McNaught said: "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."