A landslip between Haywards Heath and Brighton means that some lines are currently closed.
Southern Rail said trains may be cancelled, severely delayed by up to 60 minutes or revised.
The landslip occurred in the Hassocks to Burgess Hill area, and Network Rail is trying to resolve it.
Fewer trains than usual will run between Gatwick Airport and Brighton, and journeys between these stations will take up to 60 minutes longer than normal.
Where possible, trains will divert to run via Horsham and the West Coast.
If you are travelling between Littlehampton and London, these trains will continue to run via Horsham rather than via Worthing / Brighton. As such, they will not call at stations between Angmering and Hove. If you are travelling to or from these stations, you can travel to Barnham and change there for a train to your destination.
CCTV released after teenage girl verbally abused and spat at in Gosport
Worthing care provider fined thousands of pounds
Award winning Portsmouth historic dockyard
New Red Panda spotted on the Isle of Wight
Record numbers at Sussex food banks
Appeal for information following serious collision in Climping
Arrests made to tackle anti-social behaviour in Littlehampton
Portsmouth University provides free meals to students to help with stressful exam period
South East: Ambulance service staff have conflict resolution training
Portsmouth: Residents can share their views on amenities at the Hard Interchange
On Air