London Underground workers are staging a 24-hour strike in a bitter dispute over the sacking of a colleague.
Members of the Rail, Maritime and Transport (RMT) union based at London Bridge station, one of the busiest on the network, will walk out for 24 hours from 10pm.
The union is protesting at the sacking of a member of the station staff and disciplining of two others over an incident involving a fare dodger.
The RMT maintains the staff should have been praised for tackling a violent passenger. London Underground disputes the union's version of the incident.
The union is preparing to ballot all its station staff members on LU in an escalation of the row.
RMT general secretary Mick Cash said: "Our members are standing up 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 take this industrial action.
"Despite the spin from LU, the facts of this case remain the same. ?It is about a fare dodger stopped and asked to pay his way, who then turns violent and attacks staff doing their job.
"LU's message to thugs is that you can keep on fiddling your fares and lay into staff with impunity, and it will be the frontline tube workers who get the sack. It is outrageous.
"This is one of the most appalling abuses of the LU disciplinary procedure that RMT has ever come across.
"This was a shocking, violent incident and those that bore the brunt of it should have been supported and commended by the company.
"Instead, they have been sacked or disciplined in what is the most appalling multiple miscarriage of justice."
Brian Woodhead, operations director for London Underground, said: "The version of events described by the RMT union is simply untrue.
"CCTV footage from a number of different angles clearly shows a staff member displaying unacceptable conduct when dealing with a member of the public.
"The member of staff was dismissed following a full and thorough investigation.
"?We have subsequently invited the general secretary of the union to view the footage himself but our invitation has not been accepted.
"We intend to keep London Bridge station open throughout the strike."