Part of the Northern line has now been suspended this afternoon.
Passengers are facing worsening disruption as delays have escalated on the route.
Commuters are being advised to check before they travel and allow extra time for their journeys.
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Northern line
There is no service between Camden Town and East Finchley while engineers deal with a signal failure at Archway.
There are severe delays between East Finchley and High Barnet.
Transport for London (TfL) said tickets are being accepted on London Buses.
There is a good service on the rest of the line.
