Rail services into London from a number of Sussex stations will be disrupted until 1pm, according to National Rail.
All lines have reopened between East Croydon and Gatwick Airport, following an earlier fault on a train between these stations.
However, while service recovers, trains may be cancelled or delayed by up to 15 minutes, National Rail said.
Disruption is expected to continue until 1pm, National Rail added.
This will affect Gatwick Express services between London Victoria and Gatwick Airport.
Southern services between London Victoria and Bexhill / Bognor Regis / Brighton / Eastbourne / Portsmouth Harbour have also been impacted.
Thameslink between London Bridge and Horsham, and also between Bedford and Three Bridges will be delayed by up to 15 minutes.