London bus workers on Saturday raised the threat of industrial action during the Olympics after voting to strike if they do not receive a £500 bonus.

Members of the Unite union supported industrial action by a ratio of more than nine to one, on a 38% turnout. The union is seeking the bonus, which will cost £14m, for the 20,000 bus workers it represents, claiming that all other transport workers are being paid a premium for working during the event. At least 800,000 extra passengers are predicted on buses during the Olympics.

The ballot allows the union to name a date for a strike, but it has yet to do so. Unite said it was giving the 21 bus companies a final opportunity to consider the "landslide" ballot result before announcing possible strike dates early this week.

Unite official Peter Kavanagh said that negotiations had been going on for a year: "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. Our members want the Games to be a success, but their patience has run out. Every single London transport worker will receive a reward to recognise their major contribution to this historic occasion except for bus workers. This dispute could be brought to an end now if the bus companies and TfL [Transport for London] have the will to provide the relatively small amount of money compared to the billions being spent on the Games."

Leon Daniels, TfL's managing director of surface transport, said: "I am not surprised that 70% of London's bus drivers did not vote for strike action. That is because about 70% of local bus services are not affected by the Olympic Games."

Daniels referred to the potential bonus as "a further multi-million pound burden to the hard-pressed farepayers and taxpayers of London".

The turnout is likely to rile London mayor Boris Johnson, who has called for a minimum voting threshold of 50% to be imposed on strike ballots. Unite argues that Johnson was re-elected as mayor last month on the same turnout.

Last week the RMT union reached a deal for Underground workers that will see employees receive up to £850 each for working during the Games.

The bus operators argue that they cannot afford extra payments under the strict terms of their contracts with the mayor's transport authority. TfL has said the payments are a matter for operators.

It is understood that the conciliation service Acas has not been contacted but it is one option for bus operators and Johnson if they want to head off the immediate threat of strike dates. Under the terms of the 1992 Trade Union and Labour Relations Act, Unite cannot target the Olympics straight away for negotiation leverage.

According to the act, Unite now has 28 days to stage industrial action in order to keep its mandate alive. If that does not happen, its members will not be able to strike during the Olympics or Unite and would have to launch a new ballot.