The next Tube strike will take place on 6 February unless a dispute with Transport for London (TfL) can be resolved, the RMT union has said.
The PA reported today that the RMT had warned industrial action will be "escalated" if TfL cannot resolve the dispute.
The threat came days after Tube workers brought the capital's transport network grinding to a halt as they walked out over ticket office closures, costing the capital's businesses tens of millions of pounds.
Yesterday RMT balloted its members over another walkout, saying there had been a "breakdown in industrial relations" with London Underground.
"LUL management have gone back on agreements over the Night Tube and staffing levels and actively attack our reps for carrying out union duties," said RMT general secretary Mick Cash.
"RMT members and the union will not stand by while an intransigent management attempts to ride roughshod over Tube workers."
TfL has been contacted for comment.
This breaking news story is being updated as more information emerges - please refresh the page for the most recent version.