The threat of a strike on London Underground came a step closer when union leaders announced that workers will be balloted for action from next week.

The Transport Salaried Staffs Association said its 2,500 members on the Tube will vote on whether to launch a campaign of industrial action in a row over safety and staffing levels.

The Rail Maritime and Transport Union, which is also involved in the row, has not yet set a timetable for balloting its 5,000 Tube members but has warned of a strike vote unless the dispute is resolved.

Gerry Doherty, general secretary of the TSSA, said the union hoped to resolve the dispute with London Underground (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 following deadlock over a raft of safety issues including the closure of ticket offices, lone working and the use of agency staff.

Transport for London said the negotiating process had not broken down, adding it was "unreasonable" to curtail it by threats of strike action.

London mayor Ken Livingstone has warned the unions that going on strike would not resolve the dispute.