London Underground strikes will be "escalated" next month unless a dispute over job security and ticket office closures is resolved, the RMT union has said.
The rail union warned industrial action will increase from 6 February and also revealed a ban on overtime will be extended as part of a "new phase" of action.
The announcement came just minutes after Acas said further talks would be held to attempt to resolve workers' grievances.
The conciliation service announced that London Underground agreed to attend a meeting on Monday with the RMT and the Transport Salaried Staffs' Association (TSSA), whose members walked out at the beginning of this week, causing mass disruption across the tube network.
Talks were held at Acas last week and into the weekend but ended without agreement.
The unions are campaigning to have jobs reinstated and ticket offices reopened, saying cuts agreed by former London mayor Boris Johnson have gone too far.
Additional reporting by Press Association
