Unions have warned they could ballot more than 7,500 London Underground staff on plans to strike over staffing and safety issues.
Unions have warned about the closure of ticket offices
The Rail Maritime and Transport Union (RMT) and the Transport Salaried Staffs Association said workers faced an "unacceptable" attack on safety issues.
Last year they complained about ticket office closures and staff shortages.
A Transport for London (TfL) spokesman said the unions' ballot warning was "completely unreasonable".
Workers last went on strike in September in a dispute linked to the collapse of maintenance firm Metronet.
The move caused widespread travel chaos in London.
The RMT and the Transport Salaried Staffs Association (TSSA) said the latest move came as station workers and drivers were at increased risk under plans to close 40 ticket offices and reduce opening hours.
They also have concerns about staff working alone and the use of agency and security staff, and plans to introduce "mobile supervisors" at stations.
The RMT and TSSA claim LU plans to use sub-contracting personnel to staff the Tube station at Heathrow's Terminal 5, due to open in March.
The policies were put on hold by LU after protests but not withdrawn, the unions said.
RMT general secretary Bob Crow said: "Each of these issues is serious in its own right, but together they amount to a fundamental and unacceptable attack on staffing across the network which strikes at the very heart of its safe operation."
Gerry Doherty, general secretary of the TSSA, said: "Tube users should be as appalled as our members at the number of safety issues outstanding."
A spokesman for TfL said: "All of the issues they claim are already being addressed in the normal way... RMT and TSSA should be using the agreed procedures, not threatening strike action."
The unions said they had not yet decided on a timeline for holding the strike ballot and were waiting for LU to respond.