Tooting Bec Station remains closed following an electrical fault this morning.
Firefighters were called to the busy commuter station in Tooting Bec Road, Tooting, at 4.45am to a fire alert.
They found there was a problem with an electrical instillation in the station and moved swiftly to turn the power off to prevent fire spreading.
At the time the station was not open, with only a supervisor and another member of staff present.
Crews left at 7am and repairs are being carried out by Transport for London.
A TfL spokesperson said: "The Northern line had a short suspension from Morden and Stockwell for about 20 minutes until just before 8am because of a power issue.
"It is now fully running with just a few minor delays."
The entire line is now running but Tooting Bec Underground station is still closed.
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