A landslip has closed part of the line between Hastings and London Charing Cross.
In a tweet, Southeastern, which runs the route, said services cannot run between Robertsbridge and Etchingham due to the landslip, which has blocked both lines.
Replacement buses have been ordered and will run between both these stations, Southeastern said.
Disruption is expected to run until around 7pm.
Today (Friday, December 20) has seen heavy rain and flooding affecting train services and roads across the county.