Unions have threatened fresh strike action that could impact the London underground's Night Tube service this Christmas.
The RMT announced on Tuesday 17 October that they were preparing for a fresh ballot to decide whether to strike during one of the busiest times of the year for the Tube.
The union accused the London Underground of backtracking on an agreement that would see Night Tube drivers move onto a five day a week employment schedule.
Around 40% of the drivers employed for the Night Tube, which runs along the Victoria, Central, Jubilee, Northern and Piccadilly lines, are part of the unions.
The introduction of the Night Tube was delayed by almost a year before if finally launched in August 2016 due to driver disputes.
Mick Cash, the RMT leader, 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."
Two weeks ago, a planned strike by the Aslef union was called off after progress was made in talks, but they warned that the industrial action could be restarted if talks show no further progression.
Nigel Holness, Operations Director for London Underground, said: "Successful Night Tube services have been running for over a year in line with agreements reached with the unions.
" We have already met them on this issue, and acknowledged their concerns, therefore the threat of a ballot is disproportionate and unnecessary."