The threat of a Tube strike is looming after talks to avert a walk out broke down.

Unions say London Underground broke a promise to stop using agency staff in stations.

The Transport Salaried Staffs Association (TSSA) and the Rail Maritime and Transport Union (RMT) are in dispute with Tube bosses over safety standards.

TSSA claim agency workers are not properly trained, but Transport for London (TfL) insist the policy is temporary and condemned the threat of industrial action as "unreasonable".

Unions are concerned about staff working alone and plans to employ "mobile supervisors" at stations.

They also say safety will be compromised if, as proposed, 40 ticket offices are closed and opening hours reduced.

Bob Crow, RMT general secretary 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."

"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 will ballot its members over strike action and the RMT plans to do the same.

TfL said the issues raised should be addressed through negotiation and strike threats are unreasonable.