There are severe delays on services in and out of London Victoria and Liverpool Street stations due to signalling problems.
At Liverpool Street, a fault with the signalling system means trains may be delayed by up to 30 minutes, cancelled or revised. The disruption is expected to continue until 4pm.
National Rail said a fault with the signalling system between Victoria and East Croydon has resulted in trains having to run at a reduced speed on some lines. Trains running between these stations may be cancelled or delayed by up to 20 minutes, the rail operator said. The disruption is expected until 2pm.
"Network Rail have advised us of a fault with the signalling system at London Victoria," National Rail said.
"The fault occurred is affecting a particular signal which remains red. As a result, train drivers will begin to liaise with the signaller to ensure that it is safe to proceed onwards.
"This fault is affecting some services from London Victoria heading towards Brighton. Services in the reverse direction will run as normal. Engineers have arrived on site as of 9:36am, and are currently working to restore the affected signal. Progress updates will be provided once received from Network Rail.
"You are advised to allow extra time to complete your journey and to check the journey planner prior to travelling this morning, as services are likely to be cancelled or altered."