The long-awaited all-night Tube service in London will be launched on August 19 on two lines, Mayor Sadiq Khan announced.
The 24-hour trains were due to start last year but no agreement was reached with unions over pay and conditions, which led to strikes.
The first all-night Tube services will run at weekends on the Central and Victoria lines. Night services on the Jubilee, Northern and Piccadilly lines will follow in two separate phases later in the autumn as new Tube drivers complete their training and final preparations are made.
Around 200 part-time drivers are currently taking part in a 14-week training programme for the new service.
The mayor said: "The Night Tube is absolutely vital to my plans to support and grow London's night-time economy - creating more jobs and opportunities for all Londoners. The constant delays under the previous mayor let Londoners down badly.
“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.”
The Rail, Maritime and Transport union is still in dispute with London Underground over conditions for engineering workers linked to the new service. An industrial action ballot is being held.
General secretary Mick Cash said: "There are major unresolved issues in relation to the Tube Lines engineering workforce and it is imperative that the company now moves quickly to address the points at the core of that dispute.
"RMT also still has major concerns over the safe running of the Night Tube and there are unresolved issues on the detailed safety case that will have to be agreed through the health and safety machinery."
London's Transport Commissioner, Mike Brown, said: "It is good news for London that the first ever all-night Tube service will be ready on August 19. More than half a million people use the Tube after 10pm on Fridays and Saturdays, and the introduction of the Night Tube, which will support London's businesses and jobs, is a historic step in our modernisation of the Underground and our work to support London's economic growth."
Labour's London Assembly transport spokeswoman, Florence Eshalomi, said: "This is a very welcome announcement and shows what progress can be made if you have a mayor willing to put in the work. In a matter of weeks Sadiq Khan has managed to get the Night Tube back on track after almost two years of dithering and delay under the previous mayor, Boris Johnson."
Keith Prince, Conservative transport spokesman on the Assembly, said: "It is disappointing that the Mayor, who had a chance to be magnanimous and thank his predecessor for all the work he did to make the Night Tube happen, has instead chosen to attack him.
"Given Sadiq Khan's union paymasters have already delayed the launch of the Night Tube by 11 months, it's no surprise that he's trying to blame Boris Johnson."