UPDATE At about 6.30pm, the repairs were said to have been completed and all lines were able to be used again.
Southern warned that delays, disruption and cancellations could continue until the end of the night tonight (Saturday 5 July).
A landslip has disrupted trains on the mainline between London and Brighton and has affected east and west coastway services as well as other Sussex destinations.
The landslip happened north of Gatwick, in the Horley area, yesterday evening (Friday 4 July).
Train operators have warned passengers to expect delays and cancellations for the rest of the day today.
All Gatwick Express services between Brighton and London Victoria have been cancelled as have Thameslink services.
Southern said: “A track defect affected the area earlier today and Network Rail’s engineers attended the site to repair the affected section of track.
“There are various different types of defects and the first advice usually comes from drivers or other staff travelling through the area.
“Response staff will check the location which has been specified and find out whether the track is still supported correctly, whether the rails themselves are in the usual condition and if there are any problems with the fixings and metalwork which attach the rails to the sleepers.”
No further information was given about the nature of the landslip but many trains have been running a lower speeds during the heatwave because of concerns about potential problems.
Network Rail urged customers to check with their train operator before setting off.
