Trains will be heavily disrupted into the afternoon after a person was hit by a train between London Victoria and East Croydon.
Although the line from London Victoria towards East Croydon has now reopened, services will remain heavily disrupted while trains and crew are moved to the correct locations, a Southern Rail spokesman said.
Trains are expected to be cancelled or delayed until 1pm, he added.
The spokesman said: “The line priority will be given to the trains which have been trapped during this unfortunate incident.
“Please delay travelling in this area until later if possible.
“Please check customer information screens and journey planners prior to travelling, as these will contain up to date service information.”
Southern and Gatwick Express tickets will be accepted on Thameslink services, London Buses between Victoria, East Croydon and Sutton, London Underground via all reasonable routes, Tramlink on any reasonable route, Southeastern between Victoria and Beckenham, South Western Railway on all reasonable routes, Metrobus on all routes, London Northwestern Railway between London Euston and Tring (for Wembley Central, Harrow and Wealdstone, Watford Junction, Hemel Hempstead and Berkhampstead, London Overground between Watford Junction and London and Virgin trains between Euston and Milton Keynes.
The spokesman added: “Gatwick Express services are suspended in all directions.
“Very limited service is running between Victoria and East Croydon in either direction.
“The partial reopening of the lines will prioritise trains which have been trapped.
“Southern services will be diverted to London Bridge where possible, and some services will be terminating short of their destination.
“Services between East Croydon and Milton Keynes Central are suspended in both directions.
“Thameslink services through East Croydon may be also be disrupted.”
There will be a much wider impact to services as many routes cross through the affected area, according to Southern.
The spokesman urged passengers to check before travelling and to delay journeys until later.
He added: “Southern work closely with The Samaritans and support the work that they do.
“If you or someone close to you has been affected by this incident or other, there is always someone to talk to.
“Contact the Samaritans on 116 123 free at any time, on any phone. Help and advice is only a phone call away.”