Service has resumed on the Piccadilly, Circle, District and Hammersmith & City Lines after they were suspended following a signal systems failure this morning.
However, severe delays were affecting all four lines.
Apologies for the disruption to your journey. Our engineers are on site working fast as they can to resolve this issue.— Circle line (@circleline) June 26, 2017
A spokesman for Transport for London (TfL) said the problem was due to a local power failure in the Earl's Court area, which knocked out power supply to the control room.
TfL is switching power over from the local supply to its own supply, but it said the process will take "some time".
In a tweet, it "strongly advised" customers to use alternative routes.
Customers are strongly advised to use an alternative routes. To re-plan https://t.co/7vvV4ibd4O Your tickets will be accepted on local Buses— District line (@districtline) June 26, 2017
Meanwhile, the London Overground was hit by severe delays between New Cross Gate and West Croydon and Crystal Palace while TfL fixed a faulty train at Brockley.
This breaking news story is being updated as more information emerges - please refresh the page for the most recent version.