The problems come just one day after news that drivers on the Jubilee line are to be balloted for industrial action in a row over working conditions and allegations of bullying.
Members of the Rail Maritime and Transport union on the Jubilee line will vote in the coming weeks on whether to launch a campaign of industrial action.
RMT general secretary Bob Crow said: "Our members will not sit back and allow the misuse of procedures to bully and intimidate our members, and they are are sick of the gradual decline in working conditions."
John Doyle, Jubilee Line general manager for London Underground, said: "The unions have informed us that they are in dispute with us over what they perceive as a breakdown in industrial relations.
"We are keen to understand their concerns and so will be meeting with the union later this week to discuss these issues in greater detail."
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Before the final declaration on Friday, the Conservative candidate looked set to lose the City Hall race to Labour's Sadiq Khan.
The Tooting MP has been the frontrunner in the counting process ahead of Tory challenger Zac Goldsmith.
It's not just London where elections have been taking place today. There were council elections taking place across the home counties.