Leaders of hundreds of London Underground engineers, electricians and other workers have rejected an offer of £850 for working during the Olympic games because of "unacceptable conditions" attached to the deal.
Unite, which represents engineering, electrical, power control and management workers, said the company was demanding "unlimited flexibility" for an indefinite period of time after the Games have finished.
Regional officer John Morgan-Evans said: "Unite was prepared to accept the deal, but the strings attached are totally unacceptable. It would be irresponsible to accept this deal as it would put our members' health and safety at risk.
"Our members have been asked to agree to limitless changes to hours and place of work at short notice, not only during the games but for an undefined and potentially indefinite period after the games are over.
"We can't give London Underground a free hand to make such changes outside of the normal processes of negotiation. This has never been just about reward. It is also about protecting the health and family life of our members."
The same offer has been made to the Rail Maritime and Transport union, which has yet to decide whether to accept it.
Unite added that London bus operators are continuing to refuse to meet the union to discuss its claim for a payment of £500 for the Olympics period on behalf of 28,000 bus workers.
The union warned it will be balloting its members with a view to potential industrial action.
Peter Kavanagh, Unite's regional secretary for London, said: "London's bus operators are not taking their responsibility for transport during the Games seriously by refusing to even meet with Unite to discuss a reasonable Olympic payment.
"Bus workers will be central to the transport infrastructure during the Olympics and their importance should be recognised."