The threat of industrial action on the Tube has moved a step closer after union leaders announced that workers will be balloted for action.
One of the safety concerns of Tube workers is lone working
The Transport Salaried Staffs Association said its 2,500 members will vote on whether to strike in a row over safety and staffing levels.
Its workers will vote from 15 February to 11 March, but the Rail Maritime and Transport Union has not yet set a date.
Transport for London (TfL) said threats of strike action were "unreasonable".
Union leaders from the Rail Maritime and Transport Union (RMT) confirmed it is yet to set a timetable for balloting its 5,000 members working on London Underground (LU) but has warned of a strike vote unless the dispute is resolved.
Gerry Doherty, general secretary of the Transport Salaried Staffs Association (TSSA), said the union also hoped to resolve the dispute with LU through normal negotiations.
He said: "We now have a window of opportunity to do just that. Industrial action is always the last resort.
"I am confident our members will support us if that is what is needed to resolve this dispute, but the ball is now firmly with London Underground."
The ballot has been called due to a lack of progress regarding a range of safety issues including the closure of ticket offices, lone working and the use of agency staff.
TfL said the negotiating process had not broken down.
Meanwhile London mayor Ken Livingstone warned the unions that going on strike would not resolve the dispute.