There are severe delays on a number of Tube lines following a "signal systems failure".
According to Transport for London (TfL), the Piccadilly, Circle, Hammersmith & City, and District lines are all experiencing delays.
The Piccadilly and District lines were completely suspended on Monday morning (June 26), while engineers worked as "fast as" they could to fix the problem.
Tube users are being advised their tickets will be accepted on London Buses, London Overground, DLR, Heathrow Connect, SWTrain and other services.
Both lines resumed service at around 10.30am, but severe delays remain.
Other lines have posted on Twitter to warn those travelling.
Commuters have posted on social media to complain about being stuck on trains, with one Twitter user sharing a video of people singing Happy Birthday.
Isabel Baquero-White wrote to the Piccadilly line Twitter account, saying: "Can you please give an update. Stuck on this train for 30mins between North Ealing-Ealing Common and very late for my work at @LAMDAdrama".
Those planning to travel on the Tube are being advised to re-plan their journey and use an alternative route.
To check your journey, head to TfL's website.
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