A points failure at Rye is disrupting trains between Ashford International and Eastbourne.
The line is currently blocked at Rye in the direction towards Hastings.
A replacement bus is on the way to Rye and will call at Three Oaks, Ore and Hastings.
Network Rail specialist engineers are on their way to the site and will provide an update in due course.
Southern added: “Our Network Rail colleagues have advised us of a points failure between Rye and Hastings. Currently the line towards Eastbourne is blocked.”