Major engineering works have closed some lines in the county today (Saturday, March 9).
All routes via Lewes are shut due to the works.
Southern said services running between Brighton and Eastbourne and also between Haywards Heath and Seaford will be affected by this closure.
Replacement buses are operating for part of or all of these journeys, and journey times will be extended, the train operator added.
There will be no trains between the following stations between Wivelsfield and Lewes, Falmer and Lewes, Seaford and Lewes and Polegate and Lewes
Between Brighton and Falmer a shuttle train will be replacing normal service, running every 40 minutes, Southern said.
Buses will replace trains between Haywards Heath and Polegate / Eastbourne via Lewes and between Brighton and Seaford and Eastbourne via Lewes.
The Newhaven level crossing barriers will also remain closed to all road traffic until Monday (March 11).