A further 300 London bus workers are to be balloted for strike action in a row over pay, the Unite union has said.
The London Transit drivers and engineers are said to be angry over a 3% pay offer, which they say amounts to a real-terms pay cut.
About 1,600 drivers with the firm in central, south and west London were already being balloted for industrial action on the same issue.
French state-owned RATP Group, which runs London United, has been contacted for comment.
The ballot opened this week and will close on 31 July.
The dispute is also over London Transit’s plans to impose an “unacceptable scheduling agreement”, Unite said.
Unite general secretary Sharon Graham said: “RATP is a multi-billion pound company and can more than afford to put forward reasonable pay offers for its London Transit and London United workers.”
Ms Graham also called on the firm to “properly consult with Unite to ensure that its scheduling agreements are fit for purpose”.
In addition, about 100 engineers based at eight garages are being balloted for strikes until 1 August over a 3.6% pay offer. They are employed by London United.
Unite said the workers rejected the pay offer made in December as it did not match the rate of inflation at the time, which was 5.2%, according to the Office for National Statistics.
This dispute follows previous strikes in recent years by drivers for London Transit and the operator Abellio.
A Transport for London spokesperson said: “We encourage Unite and London United to work together to come to an agreement and avoid disruption to Londoners.”