Bus passengers across London will face disruption this month because of a series of strikes by drivers over pay.
Unite union members, employed by French bus company RATP, walked out on Wednesday, with further strikes planned for 17, 24 and 31 March.
The union has said the action would have an increasing impact as schools reopen and more people return to offices and shops.
Talks aimed at resolving the dispute have failed to break the deadlock.
RATP operates three subsidiaries across the London bus network: London United, London Sovereign and Quality Line.
Unite regional officer Michelle Braveboy said: "Bus drivers are not taking strike action during a pandemic lightly, this is a last resort as RATP has refused to enter into meaningful negotiations.
"RATP is using the Covid pandemic as cover for attacking drivers' pay and conditions. It is expecting drivers to do the same work for lower pay."
In response, Transport for London said: "We urge Unite the Union and RATP Dev London to find a resolution to this dispute as soon as possible.
The statement added: "If further strike action goes ahead, we will be doing everything possible to reduce the disruption, but customers should check before they travel and allow more time for their journeys, while remembering the Government's requirement to continue to stay at home if they can."
Meanwhile, hundreds of bus drivers in Manchester are in a second week of strikes in a row over working practices.
The action follows proposed changes by the Manchester-based Go North West at their city centre depot in Queens Road.