A speed restriction is in place between Seaford and Lewes.
Southern said train services running to and from these stations may experience delays.
Disruption is expected to last until 8.30pm.
A spokesman said: “A speed restriction over defective track between Seaford and Lewes means trains may be delayed.
“We anticipate disruption will continue until 20:30.
“Train services leaving Seaford towards Lewes and Brighton may reach Brighton 10 minutes later than expected.
“Network Rail has confirmed a rail defect at Southease, affecting the line towards Brighton.”
Visit the Southern website for more details.