Rail services connecting Littlehampton with London Victoria have been suspended in both directions following a technical fault.
A track circuit failure has been identified as the cause of the disruption.
Track circuits form part of the signalling system used to monitor the position of trains on the network, and when one develops a fault, other services cannot proceed along the affected section.
The halt to operations is anticipated to continue until 1pm.
Passengers holding tickets for the affected route are being permitted to travel on alternative services at no additional cost.
Valid alternatives include any Southern Railway service running between Littlehampton, Hove and Brighton, Thameslink connections from Brighton to London, Gatwick Express services between Brighton and London Victoria, and London Underground routes between London terminal stations.
Those travelling from stations between Littlehampton and Hove heading toward the capital have been advised to board a West Coastway service proceeding to Brighton and change there for connections to London Victoria.
The train operator has indicated that passengers should allocate an additional 30 minutes to complete their intended journeys and consider using alternative routes where possible.
Services operating from Brighton in the direction of London may experience delays of up to 10 minutes.