Frustrated passengers are expected to be disrupted for hours due to a ‘track defect’ at Burgess Hill.
Hundreds of people are currently stuck at Haywards Heath station because of the issue.
A spokesman for Southern said: “The 06:46 Southern service from Brighton to London Victoria reported hitting an obstruction on the track on approach to Burgess Hill station.
“Network Rail have a team on site now and have confirmed that there is a track defect and not an obstruction as previously reported.
“There is a speed restriction in place over a defective track between Brighton and Haywards Heath.
“Consequently, trains between these stations may be cancelled, delayed by up to 15 minutes or revised.
“Disruption is expected until 12:00pm.”
Customers are advised trains may be cancelled or delayed.