Drivers on London Underground are to be balloted for industrial action in a dispute linked to the Night Tube.
Scores of members of the Rail, Maritime and Transport union will vote in the coming weeks on whether to launch a campaign of action.
The union accused LU of failing to honour an agreement on career progression into full-time jobs for drivers on the night service, which was launched just over a year ago.
RMT general secretary Mick Cash said: "Staff are rightly angry and frustrated that an agreement that would have allowed Night Tube drivers to progress to full-time driver jobs is being flouted by the company.
"That is wholly unacceptable and has left RMT with no alternative but to declare a dispute and begin preparations for a ballot.
"RMT remains available for talks but we will not sit back while our members are denied the career opportunities they deserve."
A planned strike by drivers in Aslef was called off earlier this month after talks over demands for a four-day week and a reduction in weekend duties, which the union said had been agreed as part of the deal to set up the Night Tube.