The biggest rail union has ended talks aimed at averting a strike by London Underground workers, dashing hopes of a last minute deal.

But a second union is still meeting the company to discuss a dispute over jobs and ticket office closures.

Officials from the Rail, Maritime and Transport union (RMT) and the Transport Salaried Staffs Association (TSSA) met with London Underground under the chairmanship of the conciliation service ACAS for a fourth day of talks.

But the RMT left the meeting without any sign of progress being made.

TSSA General Secretary Manuel Cortes remained at ACAS, making it clear he was determined to continue trying to break the deadlock.

Members of the two unions are due to stage a 24-hour strike from 6pm on Sunday.

A TSSA source said: “We really are more concerned about working out how to undo the staffing and safety problems Boris Johnson and George Osborne bequeathed to the Tube.

“Resolving these issues so staff can work in a safe place and know they’ve got procedures in place to protect and keep safe their passengers are what concern our overworked, over-tired and over-abused Tube staff. That takes talking.”