Network Rail is urging commuters rail passengers in south London and Sussex to plan ahead of Easter weekend and check timetables due to engineering works.
Southern and Thameslink services will run an amended timetable across all routes from Friday 19 to Monday 22 April.
On Friday and Saturday, Southern will have a reduced service into London Victoria with some services diverted into London Bridge. Gatwick Express services will not run.
On Sunday and Monday, Southern services between Victoria and Sutton via Crystal Palace will operate between West Croydon and Sutton only. Gatwick Express will run half hourly.
The major investment projects this Easter include:
• Laying new track between Thornton Heath and Selhurst from Friday 19 to Saturday 20 April.
• In Balham, engineers are replacing switches and crossings, the moveable sections of track that guide trains from one track to another allowing them to cross paths.
They are part of Network Rail's railway upgrade plan, a multi-billion-pound investment in the rail network to improve passenger journeys.
Thameslink services from the south will terminate at either London Bridge, London Blackfriars or London St Pancras.
From the north, Thameslink services to and from Bedford, Luton and St Albans will terminate in the high level platforms at London St Pancras.
Thameslink Kent services will run between Dartford and Rainham only.
On Sunday there will be no Thameslink services between Blackfriars and Wimbledon via Sutton.
A breakdown of how journeys will be affected can be found by visiting nationalrail.co.uk/Easter and following #EasterRailWorks on Twitter.