Transport for London faces significant disruption after the first of two planned 24-hour strikes by London Underground drivers got under way.
Members of the Rail, Maritime and Transport union walked out in a dispute over plans by Transport for London to introduce a voluntary four-day working week.
The two sides held last-ditch talks on Monday, but the RMT said Transport for London had “failed to provide assurances” over concerns about longer working hours and driver fatigue.
The strikes officially began on Tuesday at 00:01 BST, with very few services expected to run before 06:30 or after 21:00.
The Circle and Piccadilly lines will shut down entirely, while parts of the Metropolitan and Central lines will also be suspended.
Buses, London Overground, Elizabeth line, DLR and tram services will continue throughout the strike but are expected to be significantly busier than usual.
A follow-up 24-hour strike is planned for Thursday, with some residual disruption also expected on the mornings of 3 and 5 June.
Transport for London’s proposal for a four-day working week would be voluntary, meaning drivers would work slightly longer days but fewer hours per week and fewer hours overall.
Members of the trade union Aslef accepted the plans, but the RMT union voted to take industrial action, announcing several strike dates across the dispute.
The first round of strikes caused major disruption on the network in April, while further strikes in May were called off after talks between the union and Transport for London.
Following negotiations with conciliation service Acas on Monday, an RMT union representative said: “Despite our best efforts in ACAS talks, TfL have failed to provide assurances on our members deeply held concerns around fatigue, reduced flexibility, shift lengths and the impact these proposals could have in a safety-critical role like tube driving.”
The representative added: “We remain available for meaningful talks, but strike action tomorrow will now go ahead.”
A Transport for London spokesperson said: “It is bitterly disappointing that despite five hours of meetings with the RMT at ACAS and repeated assurances that the four-day working week proposals will remain voluntary, RMT has chosen to continue with its disruptive strike action.”
The spokesperson added: “We will do all we can to provide as much service as possible during this action.”