Thousands of bus workers are to vote on strikes in a row over payments during the Olympic Games.
Unite said it had given notice to 21 bus companies in London of its intention to ballot up to 21,000 members in the capital from next week.
The union has called for a £500 payment to recognise the "massive increase" in workload during the Games this summer, saying that 800,000 extra passengers are expected to travel on buses.
Unite pointed out that other transport workers, including those on London Underground, London Overground, Docklands Light Railways, Network Rail and Virgin, will receive hundreds of pounds in extra payments during the Olympics.
The union called on London Mayor Boris Johnson to intervene in the dispute to remove the threat of industrial action in the run-up and during the Games.
Unite official Peter Kavanagh said: "Bus workers will be going above and beyond the call of duty to make the Olympics a success. They are the only London transport workers not getting an Olympic payment for their extra work.
"The first priority for London's re-elected mayor must be to demand that bus operators agree a fair award for their workers during the Olympic Games.
"If bus workers take strike action in the run-up to and during the Olympics it will be because Boris Johnson and the bus companies failed in their duty to London.
"Boris Johnson has sat on his hands for almost a year while the bus operators have refused to even talk to Unite about their workers' extra contribution to the Olympics.
"Bus workers deserve recognition for helping hundreds of thousands of extra passengers during the Olympics and dealing with major congestion on London's roads."