London bus drivers will strike on June 22 and may stage further walkouts up to and during the capital's summer Olympics in a dispute over extra payments for working during the Games, their union said on Friday.

Any walkout during the Olympics would embarrass Prime Minister David Cameron's government, which has promised that London's transport system will not buckle under the strain of tens of thousands of extra visitors.

Bus drivers are seeking a 500-pound ($780) bonus to compensate for a "massive increase" in their workload during the Games, which start on July 27, the Unite union said.

Similar Olympics bonus deals have already been struck with staff working on the capital's rail and underground networks.

"There is no moral or economic justification for treating bus workers like second-class citizens. There is a clear precedent for rewarding bus workers for keeping London moving over the Olympics," said Unite official Peter Kavanagh.

Unless there is a last-minute deal with the 20 private operators running the capital's bus network, next week's strike threatens to cause serious problems for the 6.4 million passengers who travel daily on buses in London.

It would be the first London-wide bus strike since 1982.

Sports Minister Hugh Robertson this week ruled out using contingency funds left from 9.3 billion pounds of public funding for the Games to subsidize the bonus sought by the drivers.

Unite said the 14 million pound cost of paying the bonus to more than 20,000 drivers was easily affordable from the remaining 476 million pounds of Britain's Olympics budget.

London's transport authority, Transport for London, said it was up to the drivers and bus operators to settle the issue.

Unite said the operators, who include Stagecoach, FirstGroup and Go-Ahead, had made no effort to resolve the dispute. The operators could not immediately be reached for comment.