Trains are unable to run between Chichester and Portsmouth this morning (March 21).
Southern Rail said a fault with the signalling system at Havant had stopped trains from running between the two stations.
The rail service provider has put on a Stagecoach 700 between Portsmouth and Chichester for commuters.
Southern said other services travelling to and from Havant may also be cancelled or delayed by up to 30 minutes.
Engineers have been working through the night to try to resolve the issue but were unable to fix it.
Disruption is expected to last until 3pm.