Network Rail and Thames Water confirmed the discovery in the early hours of this morning, 19 July.
It is thought the hole has been caused by a collapsed active sewer and work is underway to design a repair plan.
The disruption is expected to last all day with Southern, Thameslink and London Overground services affected.
Paul Rutter, area director for Network Rail, said: "Following investigations, a collapsed sewer has been found at the site of the hole which was found on the railway in the Forest Hill area yesterday. Network Rail engineers have been working with our counterparts at Thames Water throughout the night to identify and fix this problem as quickly and as safely as possible."
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Danny Leamon, Thames Water's senior operations manager, said: "We're acutely aware of the importance of this rail route. Our specialist teams worked through the night, and they'll continue to work as fast as possible today to get the pipe fixed and help Network Rail get the track reopened.
"Our sewer is below the track, but it is too early to say whether the sewer pipe is the cause of the sinkhole.
"This is an extremely complex job as the pipe is in a difficult location in the tracks and six metres below the ground. Engineers hope to finish repairs later today but we're unable to put an exact time to it at this stage"
(LM/CD)
