Leaders of thousands of London Underground workers formally declared a dispute today after lack of progress in reaching a deal over payment for working during the Olympic Games.
The Rail, Maritime and Transport (RMT) union has rejected an offer of up to £500 for Tube staff, saying there were too many strings attached to the proposed deal.
The union also announced it is to ballot hundreds of administrative staff it represents at Transport for London (TfL) for industrial action over "severely restricted" annual leave during the Games this summer.
Officials claimed that leave had been banned altogether for the duration of the Games in one department, saying this would make life "impossible" for many staff, especially those with school-age children.
The RMT has struck several deals with rail companies for extra pay during the London Games, including a bonus of £500 for staff at Virgin Trains and Network Rail, £600 for London Overground workers and an agreement with Docklands Light Railway which union officials said could be worth up to £2,500.
RMT general secretary Bob Crow said: "RMT reiterates our stance that all grades of transport employees are entitled to a decent financial reward for their efforts transporting huge numbers of passengers during the Olympics and are entitled to take leave during the summer.
"Working conditions and important agreements should not and need not be attacked in order to facilitate Olympic running."
Unite is seeking a £500 payment on behalf of thousands of London bus workers for working during the Olympics.
Unions argue that their members will have to work extra or longer shifts, change working arrangements and be under increased pressure because of the huge numbers of sports fans travelling in London this summer.
London Underground chief operating officer Howard Collins said: "We have been in discussion with the unions for many months about working patterns during the Olympics and have put forward our proposals about how to fairly reward staff.
"We have scheduled further discussions with the unions on the issue today and have also agreed to continue talks at Acas later on this week should an agreement not be reached at these discussions."
Gillian Alford, TfL's director of corporate industrial relations, said: "Like all organisations, we need to carefully plan how our office-based staff work during the Games. Some office-based staff will be asked not to take leave during particularly busy periods when TfL will be working to deliver a successful Games while keeping London moving.
"Staff who normally work on rostered shifts will continue to do so during the Games and will be paid for any additional hours worked or for working during unsocial hours.
"In putting these sensible and proportionate arrangements in place, full account is being taken of any responsibilities our staff have for looking after children or others.
"We are talking to the unions about providing additional arrangements to help employees manage their 2012 leave entitlement."