A second strike is planned for 12 September
Talks have failed to prevent a planned 24-hour strike by thousands of London bus drivers in a row over pay.
About 2,500 employees of First will strike from 0300 BST on Friday, a move expected to mainly affect bus routes in the east and west of the city.
Members of the union Unite want a standard £30,000 rate of pay for drivers across the bus network.
A First spokesman said its pay offer was "reasonable" and the action would affect 14% of London's bus network.
Unite said pay disparity between London's 18 different bus operators can be as much as £6,000. Sick pay, pensions and holiday arrangements also differ, it added.
It wants all of London's 28,000 bus drivers to be paid £30,000 a year, based on a 38-hour week.
Unite industrial organiser George Dodo-Williams said: "First group are making huge profits while their workers are struggling to keep up with the rising cost of living in one of the world's most expensive cities."
The industrial action will include drivers who work for First Capital East and Centrewest London Buses, which operate under First.
First managing director Adrian Jones said the union rejected a 4% pay increase on Wednesday.
"This was despite me making clear that this offer was at the absolute limit of that which the company could afford and would not be available if strikes take place," he said.
Mr Jones said most drivers were paid more than £25,000 a year with 25% of drivers earning more than £30,000 a year for a normal working week.
He described the union's campaign to standardise pay as "unrealistic".
Unite members plan a further 48-hour walkout on 12 September.