The RMT union has told members that it will “probably need to strike again” over the proposed mass closure of Tube ticket offices with 950 job losses.
It has sent an email to activists urging them to recruit other staff to join the fight.
Flyers will be distributed across London urging members to “get set to strike”, and claiming talks have made little progress.
London Underground and the RMT are due to go to the conciliation service Acas on Friday to try to resolve the dispute.
Talks were put back a week following the death of RMT general secretary Bob Crow.
LU wants to shut ticket offices in a move towards full automation. The only exception will be a handful of information centres at major stations.
In February a 48-hour strike paralysed the network. But a second walkout was averted after LU bosses offered extended consultation and froze processing voluntary redundancy applications.
