London Underground staff on the District Line are staging a 24-hour strike amid a row over the treatment of a train driver who allegedly went through three red lights.
Transport for London (TfL) say the three incidents happened within 11 weeks.
The driver is said to have agreed to being "redeployed" to a role on a station, but union Aslef dispute this.
Passengers have been warned to expect some delays to the line on Friday.
Nigel Holness, director of operations for London Underground, said: "With so many incidents in a short space of time, despite several weeks of training and assistance, it was simply not safe for this employee to continue in a role as a driver."
TfL said the impact of the strike is "expected to be minimal" in central London, but there will be "more time" between trains on the western branch of the line and east of Tower Hill.
There will be no services to Kensington (Olympia).
Finn Brennan, of the train driver's union Aslef, claimed London Underground bosses "threatened a disciplinary hearing to make the driver 'agree' to being redeployed".
"Instead of recognising the concerns of our members, and reps, management are refusing to acknowledge that they have failed to follow their own procedures and now refuse to talk to this union."
The District Line is the fifth largest across the Tube network and serves 226 million passengers a year.