Talks aimed at averting fresh strikes by London bus workers in a row over an Olympic bonus are due to begin at the conciliation service Acas.
Members of the Unite union went on strike last month and are planning 24-hour walkouts on Thursday and on July 24, just a few days before the opening ceremony of the Games.
The union is seeking £500 for each of its 20,000 members at 20 bus companies for working over the Games period, pointing out that other transport workers are being paid a bonus. The union has added £100 to its claim for every day that a strike is held.
Ahead of the talks, Unite accused employers of acting with "gross irresponsibility" by implying workers could gain a £500 bonus under an offer previously made at Acas and rejected by the union. Unite says the £500 was immediately rejected at previous talks as it required its members to work all 29 days of the Olympics and Paralympics to achieve it.
Unite spokesman Steve Turner said: "Not only is this highly dangerous given that our members carry six million passengers daily on some of the most congested streets in the world, but it is illegal under drivers' hours and working time regulations, which stipulate minimum rest periods and maximum hours on duty.
"It is high time London's transport chiefs got serious. With only 25 days to go before the Olympics open, they must stop fanning the flames of this dispute. They know our demand for bus workers - £500 net, no corner cutting and no compromise.
"They have already insulted every one of our 21,000 members with their £12 per day offer and by telling them that they are not worth the award every other transport worker in the capital has been awarded without fuss, including those working on Boris's bike scheme who were awarded £500 just days ago.
"Now, to add insult to injury, they criticise the union for not consulting our members on an offer tabled that not only falls far short of the £500 (net) claimed, but would mean breaking the law as well as putting at terrible risk road and passenger safety at the busiest time on London's roads. Their actions are grossly irresponsible.
"The reality is that the offer made would give a bus worker just over half of the award claimed, and only then if they worked all their legally rostered shifts over the Olympic period. It beggars belief that the cartel running capital's transport system can treat a key workforce, drivers, passengers and the visitors to London with such contemptuous disregard.
"We all want the Olympics to succeed but if the bus employers cannot get serious about solving this dispute then we say to the mayor, Boris Johnson, show some leadership, it's what you were elected to do - get on one of your bikes and join these talks now."