London Underground conciliation talks over staffing levels at stations have to reach a resolution by the end of today or else the capital will need to brace itself for more Tube strike action.
The boss of the Transport Salaried Staffs' Association Manuel Cortes warned on Wednesday that progress needed to be ramped up - and fast, or more industrial action will follow.
He said: "We have now been in talks at Acas with London Underground (LU) since last month. LU are fully aware that if a deal is not reached by Friday, sadly, further strikes will follow."
The capital dealt with travel chaos earlier this month when a 24-hour walkout by members of the TSSA and Rail, Maritime and Transport (RMT) unions resulted in the closure of most Zone 1 stations.
The unions have aired concerns over the closure of ticket offices and staffing levels at stations.
While LU has agreed to new jobs, the unions haven't been satisfied by the number offered.
Talks have been going on this week to try and avoid a repeat, and the RMT has warned of further industrial action from 6 February if the row isn't resolved.
Cortes also said the mayor would have to "deal with the consequences" if no solution is found - of having "staff on our Tube whose morale has gone through the floor and who are also, understandably, angry because of chronic staff shortages".
Yesterday, a strike by RMT Tube drivers shut part of the Central line and caused reduced service on the rest of the line, with the likes of Ilford and Chingford Overground stations being particularly busy as a result.
London Underground chief operating officer Steve Griffiths said last week he hoped the talks at conciliation service Acas would "make progress towards resolving this dispute".