London Underground's two biggest unions have warned Tube bosses they could ballot more than 7,500 station staff for strike action over staffing and safety issues.

The Rail Maritime and Transport Union and the Transport Salaried Staffs Association have told Tube bosses they will start balloting 7,500 drivers and station staff unless the disputes are resolved within the next few days.

The Tube's two biggest trade unions said ticket office closures, lone working and the use of agency and security staff amounted to an "unacceptable" attack on safety standards.

"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," said RMT general secretary Bob Crow.

A Transport for London (TfL) spokesman said the unions' ballot warning was "completely unreasonable".

However, TSSA general secretary Gerry Doherty said: "Tube users should be as appalled as our members at the amount of safety issues outstanding."