An electrical fault in the Balham area is causing delays to commuters from Brighton, Hove and Portslade.
Journeys to London Victoria are running about 45 minutes late and some services may be cancelled.
Train operator Southern said that it could take until this afternoon to fix the problem which has held up passengers since between 7am and 7.30am.
The company said: “Electrical supply problems at Balham are causing delays of up to 45 minutes to journeys through the station.
“This is expected to continue until 12.30pm.
Passengers may use London Underground, South West Trains, London Overground, Thameslink, TfL Rail, Gatwick Express and London Midland services on any reasonable route.
You can follow this incident (on Twitter) using the hashtag #Balham.”