Airline passengers heading to Gatwick Airport on Sunday are being advised to allow extra time for their journeys because of engineering work on the rail line serving the hub.
Essential work means a bus replacement service will be in operation between East Croydon and Gatwick.
Network Rail says Southern, Thameslink and Gatwick Express passengers will all be affected this weekend.
Engineers will be carrying out essential maintenance to track, signalling, power supply systems and structures at more than 50 worksites. Teams will also clear excess vegetation, remove litter and undertake routine inspections.
Customers travelling between London and Gatwick Airport are being asked to travel either between London Victoria and East Grinstead for rail replacement bus connections to the airport, or from London Victoria for a direct train service which will take a slightly longer but direct route.
Rail replacement bus services will also be available for local journeys, such as to and from Purley.
Customers travelling between London and the South Coast are recommended to travel via East Grinstead, using rail replacement services to connect between East Grinstead and Three Bridges.
Lucy McAuliffe, Sussex Route Director for Network Rail, said: “The lines between East Croydon and Gatwick Airport are among the busiest in the country. We don’t close them very often and when we do we look to make best use of that access to do as much work as possible to improve passenger journeys.
“This Sunday our teams will be working at more than 50 worksites delivering a massive amount of work to ensure that trains continue to run safely and reliably on this crucial section of the Brighton Main Line.
Jenny Saunders, Customer Service director for Govia Thameslink Railway, said: “If you’re looking to travel between London and the south coast this weekend, please plan ahead and check before you leave for the station.
“The Brighton Main Line [will close] all day on Sunday, and late on Saturday night. It means journeys will take longer, may involve a bus, and are likely to be busier than normal.”
Picture: Govia Thameslink Railway