Talks to avert Tube strikes have broken down because London Underground (LU) went back on a promise to stop using agency staff in stations, unions say.
Unions are in dispute over a range of safety issues
The Rail Maritime and Transport Union (RMT) and Transport Salaried Staffs Association (TSSA) are in dispute with LU over a range of safety issues.
The TSSA says "properly trained" staff must be used instead of agency workers.
Transport for London (TfL) said use of agency staff was a temporary measure and strike threats were "unreasonable".
'Safety at risk'
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.
RMT general secretary Bob Crow said: "The concept of mobile station supervisors drives a coach and horses through the safety regime and the rule book, and passengers should be worried."
He added: "What happens when there's a fire, someone under a train or any one of dozens of possible emergencies and the person in charge is at the wrong station or even stuck between stations?"
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."
TSSA is holding a ballot of its members over possible strike action and the RMT has served notice of a plan to ballot its members.
A TfL spokesman said: "All of the issues raised by the unions can and should be addressed through the normal negotiating process.
"That process has not broken down and it would be totally unreasonable to curtail it by threats of strike action which can only lose staff pay and inconvenience the public to no purpose whatsoever."
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.