Rail services have been suspended after engineers were called in to check the safety of a tunnel.
Trains are likely to be suspended all day between Haywards Heath and Three Bridges as engineers work to fix support beams to the inside roof.
Network Rail said there is no issue with the tunnel structure itself but the emergency repairs are needed to prevent water from running onto the roof of trains.
The internal lining of the tunnel became disconnected.
Passengers have reported chaos ay Haywards Heath station with hundreds of people queuing for rail replacement buses.
The buses are being arranged to run between Haywards Heath and Three Bridges and Brighton Main Line services will run between Brighton and Haywards Heath with some services diverted to run via the Arun Valley Line.
Thousands of Leeds fans are due to travel to Brighton this evening for the Championship clash against Albion.
Network Rail said it has spoken to Leeds about the problems and said the club will try and put on extra coaches.
South West Trains, First Capital Connect and Southeastern Trains are accepting Southern tickets on all reasonable routes and the problem is set to continue throughout the morning.
If you are affected by the problems get in contact on 01273 544530 or email email@example.com.