About 2,000 changes are being made to the train timetables by the main rail company serving Brighton, Hove and Portslade.
Southern is bringing in the changes on Sunday 9 December with the high number attributed to the first phase of the rebuilding of London Bridge Station.
The company said that it would run extra trains between Brighton and London on Sundays.
A spokesman said: “Because of the large amount of changes to train times, passengers are advised to check before travelling.”
Timetables are available from staffed Southern stations and can be downloaded and printed online at www.southernrailway.com.
The transformation of London Bridge is due to start next year and take until 2018.
Robin Gisby, managing director of network operations at Network Rail, said: “This will be the most ambitious redevelopment of any London station in a generation and one of the most technically demanding projects ever undertaken on our railway.”
From next May platforms 14 to 16 will be taken out of use. This will affect some passengers from the Brighton area.
And from December 2014 until 2018 Thameslink services in and out of central London will be diverted away from London Bridge, which first opened in 1836.
The £6 billion Thameslink scheme involves “major surgery” on miles of outdated track and signalling, with upgrades from Bedford to Brighton and a fleet of new trains.
