The Night Tube will launch on the Central and Victoria lines on Aug. 19, with trains running about every 10 minutes between 12.30 a.m. and 5.30 a.m. on Saturdays and Sundays. Currently, the subway closes shortly after midnight. Three other lines — Piccadilly, Jubilee and Northern — will follow with all-night services in the fall.

Khan, who succeeded Boris Johnson as mayor this month, said, “I have made getting the Night Tube up and running a priority, and London Underground has now confirmed that services on the first two lines will launch on 19 August."

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The Night Tube was expected to open last September, but it was dogged by delays after the London Underground clashed with unions over pay and safety conditions, prompting several 24-hour strikes.

The Rail, Maritime and Transport Union has said that it accepts the Night Tube in principle but that it still has reservations. Mick Cash, the union’s general secretary, said Monday that there are still “major unresolved issues” related to its workforce and concerns about safety.

London’s population has grown substantially in recent years, putting pressure on public transportation. According to figures from the mayor’s office, the number of people traveling on night buses has increased more than 170 percent since 2000.

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London Underground has tried to ease congestion with other measures, including trials at stations in which commuters are asked to stand on both the left and right sides of the escalators — thus breaking an unstated but iron-clad rule of the Tube that says that the left is reserved for those in a rush.