Engineering works will be affecting Southern, Thameslink and Gatwick Express services this weekend.
On Saturday (June 23) and Sunday (June 24), there will be an amended late night/morning service between East Croydon and Three Bridges.
On both days, buses will replace evening and morning trains between Haywards Heath and Lewes
On Sunday, buses will replace trains between Lewes and Seaford.
There will also be no trains between Havant and Barnham on Sunday.
On the Gatwick Express, there will be an amended service until 6.20am on Sunday.
Southern said: “It is expected that journey planners will be updated by 8pm to show alterations for Sunday. On affected routes, journeys may be significantly altered and extended compared with the information shown in journey planners if you check before these times.
“There may also be other on-the-day alterations. It is therefore vital that you recheck your train as close to the time of your journey using journey planner or live departures.”