A fault with the signalling system between Chichester and Havant is causing disruption on the railway this afternoon (Friday), according to Southern.
Southern said services between these stations running towards Southampton and Portsmouth 'may be subject to delay, cancellation or alteration' until 2pm.
Explaining the issue, It added: "We've been advised of a track circuit failure, which is also affecting the level crossing in the Fishbourne area.
"The signalling system is unable to automatically verify if the next section of track past the affected signal is clear.
"Whilst this issue is ongoing, train crew will stop at the affected signal and contact the signalling centre to confirm their location, which at this point the signaller will verbally authorise the driver to continue towards the next signal. This process adds time to every trains journey, which means services can experience short delays.
"Services will be altered to reduce impact on the network, please check journey planners prior to departure if possible."