Trains between Horsham and Barnham may be cancelled or delayed due to signalling problems.
Multiple signalling faults in the Pulborough area are causing the disruption, Southern Rail said.
A spokesman said: “Trains may be cancelled, delayed or revised. Disruption is expected until 10.30am.
“Your journey time will be extended, so please check before you travel and leave extra time to complete your journey.
“If you are travelling towards Southampton, you should take a service towards Bognor Regis and change at Barnham for services to Southampton. Trains will not be calling between Horsham and Southampton.
“Network Rail engineers are on site and are working to rectify the faults as quickly as possible.”
Ticket acceptance is in place, for full details see Southern Rail’s website.