Union bosses have warned industrial action on London's Underground could cause major travel disruption when the Tour de France cycle race arrives in the capital on July 7. A previous strike was called off to allow talks, but unions said these have now broken down.
The dispute involves workers transferring to other organisations some years ago and then coming back under London Underground management, but with different conditions.
Mick Cash, acting general secretary of the Rail, Maritime and Transport union, said in a message to his members:
"I urge you all to stand firm together and support this strike action to make sure that the company returns to negotiations with a genuine approach to reach final agreement on this dispute about the outsourcing of your work and the proper reinstatement of your length of service."
More top news
Prince Harry's Invictus Games is hailed a success as it closes with star performances from the Kaiser Chiefs, Ellie Goulding and Diversity.
Levi Roots from Brixton is the UK's richest reality star and Leone Lewis also makes the Top 10, but how did other Londoners fare?
The mayor is expected to be joined by his wife when he asks local Conservatives to choose him as their candidate in the general election.