A points failure between Angmering and Littlehampton has caused significant disruption to Southern Rail services.
The fault has forced all trains travelling in the Barnham direction to be rerouted through Littlehampton.
A signal failure has resulted in delays of at least 15 minutes for passengers.
For those needing to travel between Portsmouth and Southampton, Southern Rail has arranged for services to be diverted along the Littlehampton line.
Passengers heading towards Southampton Central should travel on a Southern service to Fratton and transfer there to a South Western Railway service bound for Southampton.
Those departing Southampton should travel towards Portsmouth and change at Fratton or Portsmouth and Southsea.
The operator has introduced additional ticket acceptances to assist passengers navigating the altered network.
Valid alternatives include Southern services operating between Horsham or Brighton and Littlehampton, Bognor Regis, or Portsmouth Harbour.
South Western Railway trains between Portsmouth Harbour and Southampton Central are being accepted, as are Great Western Railway services on the same route.
Stagecoach route 500 between Littlehampton, Barnham, and Chichester has also been included in the acceptance arrangements.
Direct services connecting Portsmouth Harbour and Brighton have been suspended until further notice.
Trains running between London Victoria and Portsmouth continue to operate, with passengers advised to connect at Barnham.
Services between Barnham and Bognor Regis remain suspended.
Southern Rail confirmed that trains normally operating between London Victoria and Bognor Regis would no longer split from the Littlehampton service at Horsham.
Instead, these would run as a single unit from Horsham to Littlehampton before continuing to Barnham, where the journey would terminate.
Passengers travelling between Bognor Regis and Barnham should use Stagecoach routes 500 and 700 to reach alternative rail services at Barnham.
Those using this bus replacement should allocate an additional hour for their journey.
Passengers using alternative Southern services between Portsmouth and Brighton should allow an extra 20 minutes, while those travelling between Portsmouth and Southampton should add 15 minutes to their planned journey time.
Travellers holding tickets for the Brighton to Durrington route may now use Stagecoach route 700, which serves multiple stops between these destinations.