Trains between Angmering and Brighton and Hove are currently experiencing delays and cancellations due to a signalling problem.
National Rail said a points failure at West Worthing means that some lines are blocked.
Trains between Angmering and Hove may be be cancelled, delayed by up to 20 minutes or revised.
Disruption is expected until 9am
As a result of the points failure the 0752 West Worthing to Brighton service will not run.
The points failure at West Worthing is preventing a train from leaving the sidings based near the station.
National Rail said as a result some services that were meant to originate between West Worthing and Brighton are currently unable to run.