Southern trains from Eastbourne to London Bridge are cancelled this morning (19 July).
It follows a sinkhole opening up underneath the railway tracks at Forest Row yesterday. Network Rail and Thames Water Engineers are working to fix the problem.
Trains from Uckfield will be diverted via Crystal Palace.
Southern and Thameslink tickets will be accepted on London Buses, London Underground, South West Trains, Southeastern and London Overground services on any reasonable route.
Paul Rutter, area director for Network Rail, said, "Upon further investigation, the hole which was found on the railway in the Forest Hill area has now been identified as running above an active sewer. Network Rail engineers are working with our counterparts at Thames Water as quickly and as safely as possible to determine the best way to fix the problem.
"Because of the location of the hole we are unlikely to be able to run trains on all four tracks through to London Bridge tomorrow morning/this morning which will cause disruption to services that at this stage looks likely to continue throughout the day. Passengers are advised to check before they travel and allow extra time for their journeys, with further details to follow as soon as they are available. We apologise sincerely for the disruption and assure passengers both Network Rail and Thames Water are working as hard as possible to restore the railway to full capacity."
Passengers are advised to check before they travel and allow extra time for their journeys. You can check journeys here.
Southeastern passengers are not affected by this incident.