Tube staff have voted for a 24-hour strike over the sacking of a worker who tackled a fare dodger.
London Underground workers will walkout from 10pm on Sunday, May 7, hitting the Monday morning and evening rush hour.
The RMT action is only expected to affect London Bridge and Waterloo stations.
Union members have also threatened to stop challenging customers without a valid ticket until further notice.
Their dispute centres around a worker who was sacked after tackling an alleged fare dodger at London Bridge in November.
According to the RMT, the fare dodger assaulted three members of staff and punched a pregnant woman in the stomach.
Two other members of staff also faced disciplinary action.
Tube bosses have disputed the RMT's version of events and claim the reaction of the staff members involved “was not in line with the standards we expect from our staff”.
RMT General Secretary Mick Cash said on Thursday: “Our members have voted for action in support of their victimised colleagues at London Bridge and with LU making it absolutely clear they have no interest in the principles of natural justice we have no option at all but to move to a campaign of industrial action.
“The union executive will also continue to look at a programme of escalation in this critically important dispute.”
He added: "“London Underground should do the decent thing – reinstate our member, lift the disciplinary action and send out the message that fare-dodging and violence will not be tolerated and that staff in the front line will get the support and assistance that they need.”
Brian Woodhead, Operations Director for London Underground, said: “We conducted a thorough investigation into this incident, including an extensive review of CCTV footage from the station.
"Following a full disciplinary process, the decision to dismiss this member of staff has been upheld.
“Our investigation clearly showed that no member of staff was assaulted, nor was the incident prompted by staff defending a colleague.
"We take a zero tolerance approach to violence against our staff but we also expect our employees to adhere to acceptable standards of conduct and behaviour when dealing with members of the public. In this case, those standards were not met.
"We urge the RMT to withdraw this unjustifiable action which will only result in needless disruption to Londoners.”