Members of a rail union have voted to go on strike on the London Underground (LU) over safety concerns.
The strikes will be announced on Friday
The Rail Maritime and Transport (RMT) union said the action was in protest at reduced staffing levels which it claims will affect safety.
Earlier this month the Transport Salaried Staffs Association (TSSA) union voted to take strike action.
Transport for London (TfL) said these issues have nothing to do with safety and there is no reason for a strike.
The RMT's general secretary Bob Crow said the union would announce the strike dates on Friday morning.
The unions, which represent 7,500 station staff and drivers, said safety standards were at risk under plans to close 40 ticket offices and reduce opening hours.
They also have concerns about staff working alone and plans to introduce "mobile supervisors" at stations.
Mr Crow said: "This vote reveals the strength of feeling among our members that the safety and security of the network, for staff and passengers alike, will not be compromised under any conditions."
TSSA senior regional organiser John Page said: "This dispute boils down to LU wanting to put casual staff on stations in place of properly trained Tube staff."
A TfL spokesman said: "These issues have nothing whatsoever to do with safety, and not a single job is at risk.
"All of the issues raised by the RMT are already being addressed through the normal negotiating process.
"Significant progress has already been made and most of the issues have been agreed.
"There is simply no reason for a strike, or even the threat of one, as all of the issues being raised can be resolved."
Members of the RMT working on the LU network went on strike last September in a dispute linked to the collapse of Tube maintenance firm Metronet, causing widespread travel chaos.