Planned engineering work is set to affect Thameslink, Southern and Gatwick Express services today (Sunday, July 22).
The engineering work is taking place between Purley and Three Bridges closing some lines, according to Southern, and affecting a number of services.
The Gatwick Express service between London Victoria and Gatwick Airport will be affected until the end of service today.
Southern services between London Victoria and Brighton / Bognor Regis / Eastbourne / Hastings will be running to an amended timetable.
Thameslink services between Bedford and Brighton and between Cambridge and Three Bridges will also be affected.
Southern said journey times will be extended by up to 15 minutes between East Croydon and Three Bridges.
Southern has advised people to check before they travel.