A fire next to the track between London Victoria and Clapham Junction means all lines are blocked between the two stations.
This is causing train services to be cancelled or delayed as all lined to and from London Victoria are unavailable until further notice.
Southern said it expects the disruption to continue until 12pm.
Passengers have been advised to avoid travelling to and from London Victoria and Clapham Junction this morning until further notice, and travel to or from London Bridge instead using Thameslink or alternative Southern services.
Southern expects overall journey times to be extended by up to 60 minutes.
Southern and Gatwick Express cannot run to or from London Victoria this morning. Where possible, services will divert to London Bridge instead.
Southern and Gatwick Express tickets can be used with: mutually on either service to and from London, Gatwick Airport and Brighton; Thameslink between Brighton and London Blackfriars; Southeastern between Hastings and London; London Underground between Victoria, Balham, Waterloo, Blackfriars and London Bridge; Tramlink between Croydon and Wimbledon, changing for South Western Railway services to London Waterloo; South Western Railway services between Dorking and London Waterloo; London Buses between East Croydon and Central London.
Train services will divert to London Bridge where appropriate, subject to crew route knowledge.
Gatwick Express services are suspended, London Victoria to Dorking services are suspended, as well as London Victoria to London Bridge services, and London Victoria to Epsom services.
Emergency services are currently on scene dealing with the incident near the railway.