The first day of the strike coincides with strikes announced for London Underground and Overground staff, heaping transport misery on commuters.
Unite general secretary Sharon Graham said: “The workers’ parent company RATP is fabulously wealthy and it can fully afford to pay our members a decent wage increase.
“Unite’s members play a crucial role in keeping London moving and they are not going to accept seeing their pay constantly eroded.
“Unite does what it says on the trade union tin and always defends the jobs, pay and conditions of its members. Our members at London United will receive the union’s complete support until this dispute is resolved and a fair pay offer secured.”
The strike action will affect workers based at Fulwell, Hounslow, Hounslow Heath, Park Royal, Shepherd’s Bush, Stamford Brook, and Tolworth depots.
The union said the company was offering a pay increase of 3.6% in 2022 and 4.2% next year, which it described as a real terms pay cut because of the soaring rate of inflation.
On the same day, August 19, workers on the London Underground and Overground network are also set to take 24-hour strike action in separate disputes.
The RMT union said Tube workers are set to walk out over a dispute over pensions, while Overground workers employed by Arriva Rail London will strike over pay.
It said this means around 10,000 Tube workers will walkout, and almost 400 workers will down tools on Arriva Rail London.
Meanwhile, the TSSA union said thousands of its members in General Grades and Controllers on Network Rail would also strike on Thursday 18 and Saturday 20 August.
A TfL spokesperson said: “Strikes are bad news for everyone, and we urge Unite and RATP to reach agreement and avoid the need for industrial action."
RATP, the parent company of London United, has been contacted for comment.