BUS drivers in London are taking part in a 24-hour strike as part of a campaign for a single agreement covering pay and conditions.
Members of the Unite union at 18 bus companies walked out at 00:01 GMT. About 44 out of 673 bus routes are expected to operate on Tuesday.
Unite said that with no collective pay deal there were 80 different pay rates for drivers doing the same job.
But Transport for London (TfL) said the action was "extremely disappointing".
Services will be affected from around 04:00 GMT with buses operating on Tuesday night into Wednesday morning also set for disruption, according to TfL.
Wayne King, London regional officer for Unite, said: "Strike action is the last resort. We’ve been forced into this position by the operators’ refusal to even meet with us.
Travellers have been urged to check the TfL website for the latest updates.
A spokesperson for Metroline, one of the bus companies involved, said: "Unite is seeking to move to a common rate of pay across all bus companies in London and we have challenged this on the grounds that it would be illegal for us to agree to their demand."
The bus companies involved in the dispute are: Arriva North, Arriva South, Selkent, London General, Metroline, Metroline West, Metrobus, CT Plus, London United, Abellio West London, London Sovereign, Stagecoach, Blue Triangle, Northumberland Park, Tower Transit, Docklands and London Central.