Unite said 28,000 drivers voted in favour of the move
Thousands of London bus drivers are demanding the introduction of an equal pay rate across the network.
The Unite union said 28,000 of its members voted overwhelmingly in support of the action which would bring all of London's operators in line.
Transport for London (TfL) said it was the operators' responsibility to set the pay for bus drivers.
Unite said it wanted TfL to get the operators to bring in an annual salary of £30,000 based on a 38-hour week.
Unite claimed the disparity could be as much as £6,000 for different operators. It also said sick pay, pensions and holiday arrangements differed.
Unite joint general secretary Tony Woodley said: "The tendering system is clearly exerting pressures on the employers to cut costs, which has a direct bearing on our members' wages.
"Our members have had enough and Unite is prepared to take all necessary steps in our fight to end these inequalities."
A TfL spokesman said: "Bus drivers are employed by private bus companies and as such pay and conditions are set by them.
"We are sure the trade unions and bus companies will discuss pay and conditions as part of their normal negotiations."
The union plans to hold a demonstration march through London on 24 July in support of the campaign.