Disruption on train services into London Victoria station will continue until 7pm this evening.
Due to a fault with the signalling system between East Croydon and London Victoria, train services running to and from these stations may be cancelled, delayed or revised.
Services affected by the disruption include Gatwick Express and Southern trains.
"We do apologise for the disruption earlier today caused by the signalling fault which affected both London bound lines at Thornton Heath," said National Rail.
"Our train drivers needed to stop at the affected signals and get permission from the signaller to proceed. This process was delaying trains and congestion quickly built up in this area which is one of the busiest parts of our network.
"The fault has now been rectified and we are working hard to restore our train services timetable.
"Plans are in place to reinstate 90 to 95 per cent of our services as quickly as possible with our Gatwick Express earlier cancellations first to be reinstated, train by train. Whilst this is ongoing some services are still subject to delays and short notice alterations."