The Victoria Line has been partly suspended because an engineering team poured fast-setting concrete into a signalling equipment room.
Trains are not running between Warren Street and Brixton and Transport for London (TfL) has confirmed severe delays to the rest of the line.
TfL apologised for the delays and advised that a replacement bus service is in operation.
A spokesman said tickets are also being accepted on South Eastern Trains, South West Trains, Greater Anglia, London Overground and London Buses via reasonable routes.
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