Trains between Haywards Heath, Horsham and East Croydon are delayed due to an obstruction on the line near Coulsdon.
There are delays of up to 30 minutes and will continue until further notice.
An obstruction on the line near Coulsdon and signalling problem at Balcombe are causing disruption.
Passengers may use their ticket on the 405 bus route between Redhill and Croydon.
They can also use the Southeastern services between Tonbridge, Hastings and London, as well as using South West Trains between Clapham Junction and Portsmouth.
First Capital Connect are accepting Southern passengers between London and Brighton Check the Southern and the National Rail Enquiries website for more information.