Trains between West Worthing and Brighton are being cancelled and delayed due to a signalling fault.
A fault with the signalling system means fewer trains are able to run on some lines, a Southern Rail spokesman said.
Many services between West Worthing and Brighton are being cancelled or delayed, with disruption expected until 6pm.
Network Rail are on route to repair the fault, the spokesman said.
The situation is adding further delays to disruption caused after a person was sadly hit by a train between London and Gatwick.
All lines between Gatwick and East Croydon are now open, however, due to the impact of this incident, services will remain heavily impacted, with cancellations, delays and alterations likely, the spokesman said.