Train operators are reporting delays between Brighton and Three Bridges.
A signalling problem between Haywards Heath and Three Bridges has caused delays of up to 25 minutes.
The disruption is likely to last until at least 9am today (Monday May 11), said Southern Railway.
Minor disruption is reported in Brighton Mainline, Redhill, Mainline West, Mainline East and Gatwick Express.
London-bound trains from Littlehampton via Hove may be diverted to Horsham.