Network Rail said from 12.01am to 2pm on Sunday train services will be reduced to a single line working in both direections between East Croydon and Earlswood due to work in the Coulsdon area.
As a result, train services will be limited, and are expected to be very busy.
Where possible, Network Rail is advising passengers to travel later in the day between London and the Sussex coast.
While the work takes place, a limited train service will run in the East Croydon to Gatwick Airport section of the line.
This means only one train will run every 90 minutes between London and Brighton, this will run to and from London Victoria via East Croydon and Gatwick Airport. This will be the only train operating as a ‘through service’.
Network Rail said there will be no rail service between Redhill and East Croydon.
Rail replacement buses will run between Redhill and East Croydon, and East Grinstead and Gatwick Airport.
Trains will run between Redhill and Brighton, Redhill and Ore via Lewes, and Redhill and Portsmouth & Southsea every 60 minutes.
Shaun King, route director for Sussex, said: “I’m sorry for the disruption to passengers’ journeys from this work, but only by doing this kind of job can we continue to offer reliable journeys for passengers. I’d urge passengers to check before they travel this weekend.”
Chris Fowler, customer services director for Southern, said: “Services between London and the south coast will be extremely limited on the Sunday morning while Network Rail carry out this essential work, so we advise customers not to travel before 2pm unless your journey is absolutely necessary.
“If you do have to travel, allow plenty of extra time, and please wear a face covering in crowded spaces, out of respect for others.”