All lines between Chichester and Barnham have been disrupted due to a fault with the signalling system.
Train services running through these stations may be cancelled with disruption expected until 11am.
Southern is unable to provide a train service to and from Chichester this morning (Monday, October 28).
A replacement bus service that will operate every 15 minutes between Barnham and Chichester is running.
Southern said: “You’re able to travel at no additional cost with South Western railway between Havant and London Waterloo, and with the tube between London Waterloo and London Victoria / London Bridge.
Barnham to Havant improvement work is taking place this week as planned, and buses running between Chichester and Havant will continue to run as usual.”
Southern advised passengers travelling from Barnham towards Chichester to travel on a rail replacement bus service from Barnham. Passengers travelling further should change again for a planned rail replacement bus service towards Havant.
For those travelling from Chichester towards Barnham, Southern has advised passengers to travel on a rail replacement bus service from Chichester and at Barnham they can board a train.
Southern added: “During planned engineering work in the Barnham area this week, Network Rail engineers have identified multiple signalling issues, traced back to a faulty cable. Engineers are currently working on the fault, and we’ll update you as soon as new information becomes available.”