Trains are being cancelled and delayed due a fault on a train in the Christs Hospital area, according to National Rail.
The 17:29 Bognor Regis to London Victoria train has come to a halt at Christs Hospital station and requires assistance in order to proceed onwards, a spokesman said.
Trains are currently unable to proceed past Christs Hospital in the northbound direction between Arundel and Horsham.
The spokesman said: “Assistance will be provided by the 17:11 Portsmouth to London Victoria service, which will terminate at Christs Hospital in order to attach on to the affected train, assisting it back to the depot for further assessment.”
Disruption is expected to last until 8pm.
People are advised to allow extra time for their journeys.