All trains between Lewes and Seaford have been cancelled due to a points failure.
Southern says anyone travelling between the two stations on Thursday will need to use alternative routes.
People can use tickets on “any reasonable route” to their destination. Tickets can also be used on Brighton & Hove Buses services 12, 12A and 12X between Lewes, Seaford and Eastbourne.
Disruption is expected until about 11:00 BST, however, Southern said this time could reduce or increase.
A points failure is when a section of track at a junction, which allows trains to move from one line to another, is broken.
Network Rail engineers have been sent to the site to assess the problem, according to Southern.
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