London mayor Sadiq Khan was today facing his first major strike challenge from tube workers.
The RMT union is to ballot members in a long-running row over ticket office closures and job losses.
The union claims staff shortages at stations and in control rooms poses a safety risk.
More than 800 jobs disappeared when former mayor Boris Johnson ordered the closure of ticket office at most Underground stations.
The dispute sparked two strikes which paralysed the Tube network in February 2014.
Mr Khan recently ordered a review of the closure programme.
