RMT has announced it will be balloting London Underground (LUL) staff for more strike action "over a breakdown in industrial relations".
The union also pointed to "numerous breaches of agreed machineries and agreements by LUL management". These include "not consulting or negotiating with RMT over reduced staffing levels within fleet", and "breaching our agreement on Night Tube and failing to recruit adequate additional staff to cover this work".
RMT also accused LUL of actively attacking union reps "for carrying out their trade union duties".
"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.
"That is why we are balloting our fleet staff members for industrial action. The union remains available for talks."
The announcement comes just days after the last round of industrial action, which saw Tube staff walk out for 24 hours over Sunday and Monday, causing havoc for London commuters.