The Rail Maritime and Transport Union said that it was balloting drivers on the Picadilly line in a row over working conditions and agreements.
More top news
Almost half of London's LGBT+ community have felt threatened in the last year because of their sexuality, according to a new survey.
Mr Charles died in hospital after he was chased by a police officer into a shop in Dalston and restrained.
The Labour mayor will seek a second term in City Hall.