Despite RMT backing the Night Tube pay and conditions deal last week, the union has said it is yet to see a full safety case and there is still a dispute of more than 1,500 Tube lines maintenance staff, who are currently being balloted for strike action .
General Secretary Mick Cash said: "We may have settled the dispute for LUL staff on pay and conditions but the idea that all of the outstanding issues have been resolved, not least the safety case and the Tube Lines engineers dispute, is simply not true."
The news comes as the Night Tube is believed to be set to start in August this year , according to rail union ASLEF.
Almost a year since the 24-hour service was due to start, unions and underground bosses have frequently butted heads over pay, conditions and safety.
In its letter to Transport for London , sent on Wednesday (March 16, RMT says safety concerns should be addressed and reiterated that Tube Lines staff are currently balloting to strike.
The union says maintenance staff workers are not getting the same deal as other workers on the Underground.
Mick Cash continued: "No one has seen any draft staffing rosters and those basic operational issues would be fundamental to hitting a start date this summer. There is very clearly some distance to go in terms of getting all of the components in place.
"The whole project is being constructed against a backdrop of TFL cuts and only a fool would pretend that those cuts do not have an impact on the operational model.
"RMT remains fully engaged in delivering the Night Tube and fully committed to making sure that all of our members, including Tube Lines, are properly respected and rewarded. "