London Underground workers will stage a 24-hour strike in a dispute over the firing of a colleague, the RMT union has said.

Tube workers will walk out from 10pm on 7 May.

The union claimed one member of staff was sacked and two others disciplined for "intervening to stop a serious assault by a fare-dodger" last year.

Its executive instructed members at London Bridge station not to book in for shifts, service ticket machines or challenge passengers without valid tickets between 10pm on 7 May and 9.59pm the following day.

Transport for London (TfL) stressed the action would only affect Waterloo and London Bridge stations.

RMT general secretary Mick Cash said: "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: "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.