Signalling problems between Haywards Heath and Three Bridges are preventing passengers travelling to London.
Southern Rail’s website states: “Trains at present are unable to run northbound (towards London) between these stations.
“A reduced service will also operate between Brighton and Haywards Heath.
“Buses have been requested at Brighton, Haywards Heath and Three Bridges.
“Brighton to Victoria and Victoria to Brighton services are suspended until further notice.”
Southern said tickets are being accepted on: Southeastern between Hastings and London, First Great Western between Redhill and Guildford, London Underground on any reasonable route.
Mutual ticket acceptance is in place between Southern and Thameslink.