No trains are running between Hurst Green and East Grinstead due to a landslip.
Southern said on Twitter that due to the landslip – caused by heavy rain last night – all lines between the stations were blocked.
It said disruption was expected until Sunday, March 1.
A spokesman said: “Network Rail had previously identified a landslip at the Cookspond Viaduct, between East Grinstead and Dormans, and were planning to start work to rectify this in the near future.
“However, due to the heavy rain last night, the ground moved faster than expected and the line has to be shut.
“Network Rail Engineers are to commence remedial works shortly.”
Passengers at East Grinstead, Dormans and Lingfield, travelling towards London, will need to take a rail replacement bus to connect with a rail service at Oxted, Network Rail said.
Passengers at Oxted, Hurst Green, Lingfield and Dormans for East Grinstead will also need to get a rail replacement bus.
People can use their ticket at no extra cost on METROBUS between East Grinstead and Three Bridges, where they can connect for train services to London.
Train tickets will also be valid on Thameslink trains to London, Network Rail said.