All lines between Eastbourne and Lewes were blocked after a person was hit by a train.
National Rail said the incident is affecting services between the two stations.
Lines have been reopened however trains are running at a reduced speed.
As a result of this, trains may be cancelled or delayed.
This is expected to continue until 5pm, according to National Rail.
Passengers have been advised to check before they travel.
Southern said: “Due to a person being hit by a train between Eastbourne and Lewes, all lines are currently blocked.
“Services are currently unable to run Lewes-Hampden Park in both directions.”
In a statement to passengers, Southern added: “Please delay travelling until later today. Services are suspended between Eastbourne and Lewes until further notice.
“If you do travel, you will need to use an alternative route to complete your journey. Journey times will be extended by up to 90 minutes.
“Services which operate between London Victoria, Eastbourne and Ore, will no longer split or attach at Haywards Heath. The Eastbourne/Ore portion will be cancelled and the whole train will run to Littlehampton.
“Rail replacement transport has been requested for Eastbourne and Lewes. These are currently not confirmed. When they are confirmed and on site, they will not be running to a schedule and will be controlled by station staff.
“Your ticket will be valid on the following services: Southeastern between London Bridge and Hastings; Brighton and Hove Buses on routes 12,12A and 12X between Eastbourne, Seaford and Brighton.”