Reduced services are expected until 12pm.
This is affecting trains between Milton Keynes Central and East Croydon in both directions, and between London Bridge and Beckenham Junction in both directions.
Services are expected to be delayed by up to 15 minutes or altered.
A fault with the signalling system between Gatwick and East Croydon has also closed some lines. Network Rail, who own and maintain the signalling and track, are attending the reported signalling fault.
Trains are currently unable to run between the stations.