Thousands of London bus drivers have voted in favour of strike action - raising the possibility of a crippling walkout during the Olympics.
Members of the Unite union backed industrial action in a row over extra pay for working during the Games, which begin next month.
The union argues that each of the 20,000 bus workers it represents should receive an extra £500 for being on duty over the busy period.
All other transport employees in the capital are being paid a premium for working while the event is taking place, Unite claims.
It estimates at least 800,000 extra passengers will use London's buses during the Olympics.
Unite's regional officer, Peter Kavanagh, said it has been negotiating with Transport for London (TfL) and bus operators for almost a year.
"There are only seven weeks until the Olympics and the whole world is watching London," he said.
"It's a disgrace that London's mayor, Boris Johnson, and the bus companies have allowed this dispute to get this far.
"Our members are only asking for an extra £17 a day which will just about buy you a pint of beer and a portion of fish and chips at the Olympics.
"We have given Transport for London and the bus operators almost a year to resolve this issue.
"This dispute could be brought to an end now if the bus companies and TfL have the will to provide the relatively small amount of money compared to the billions being spent on the Games."
But TfL said bus drivers are employed by private companies which deal with their pay.
Leon Daniels, TfL's managing director of surface transport, described the extra money demanded by the union as a "multi-million pound burden".
"The whole country is really entering into the spirit of the Games and the overwhelming responsible majority are pulling out all the stops to make it a huge success," he said.
"Only Unite appears to want to exploit it by adding a further multi-million pound burden to the hard-pressed fare-payers and taxpayers of London.
"We are really disappointed that the Unite leadership is encouraging its members to take this unnecessary strike action.
"Unite has already negotiated binding deals for pay and conditions with each of the London bus companies for this year and those companies are keeping their end of the deal.
"Attempting to tear up those agreements and get a new one with less than 50 days to go before the Games is completely reprehensible and out-of-step with every other provider of services in this city this summer."
Strike dates could be announced by Unite in the coming days following the ballot.