Trains will be cancelled or delayed between Littlehampton and Chichester this morning due to a signalling system fault.
The disruption is now expected to last until 2pm.
Services between Barnham and Havant have been suspended from 11.30 to 12.30pm while repairs take place, with replacement buses between the stations, Southern Rail tweeted.
Commuters are advised to check train times before they travel.
Tickets will be accepted on South Western Railway services.
A statement on the Network Rail website has described the fault as a ‘track circuit failure’, where signalling system is unable to automatically verify if the next section of track past the affected signal is clear. It states: “To work around this problem during this time, our train crew will stop at the affected signals and contact the signalling centre to confirm their location.
“Once this has been confirmed, the signaller for this route will verbally advise the driver to continue towards the next signal.
“This process will be repeated on approach towards the second signal and will resume normal line speed once away from the affected area.
“When this method of working is implemented, delays can compound quite quickly as each time a signal is talked past, delays of up to 7 minutes per train, per signal will be added to your journey.
“Network Rail technicians are on site working on the problem, and progress updates will be provided once received.”