First Capital Connect says trains between London Bridge and East Croydon are being delayed by up to 20 minutes because of an earlier problem at London Victoria.
Commuters are describing Victoria Station as 'mayhem' after the suspension of services earlier caused major delays.
Services to and from Victoria station were put on hold ahead of rush hour as police and the fire brigade dealt with the due to a ruptured gas cylinder near Battersea Power Station.
A Metropolitan Police spokesman said:
"Police were called to Battersea Power Station, Cringle Street, at 3.17pm after a suspected industrial gas canister was found.
"No residents have been evacuated. Trains are now running again. London Fire Brigade continue to work with police on scene to make the device safe. Road closures are in effect."
The 18.05, 18.20, 18.35 and 18.50 Gatwick Express services to London Victoria have been cancelled.
Southern Railway advise that tickets are being accepted on:
- First Capital Connect
- South West Trains
- London Underground
- London buses
- London tramlink
- Southeastern trains
Due to the police closing the railway line in the Battersea Park area in reaction to the discovery of a leaking gas canister, there are no Southern services arriving or departing from London Victoria.
Thousands of commuters faced delays and cancelled journeys today after flooding partly closed the mainline between Brighton and London. Services affected during this morning's rush hour because of flooding at Patcham.