The threat of a strike on London Underground was raised yesterday when talks aimed at resolving a pay dispute ended without agreement.

Rail unions were offered an extra 0.75 per cent on top of a rise of three per cent, which was imposed last year almost sparking a series of walkouts.

The dispute was taken to the arbitration service Acas today but the unions were left disappointed after their bid for at least an extra one per cent was turned down, and demands for a shorter working week, pension improvements and better travel facilities were not considered.

Bob Crow, general secretary of the Rail Maritime and Transport union, warned of industrial action unless the row was settled.

He said: "We want these issues addressed now."

Steve Grant, London district secretary of Aslef, the train drivers' union, said: "We will be knocking on the door of London Underground tomorrow and if these issues are not addressed this year we will be looking at action."

London Mayor Ken Livingstone stepped in last year to prevent strikes when he pledged the dispute would be taken to arbitration when he took control of the Tube.

The unions had been planning a series of strikes in protest at the three per cent increase but decided to hold back after the mayor's intervention.