Train passengers are being urged to check before they travel during the upcoming Bank Holiday weekend.
Some journeys are set to be affected by engineering works carried out by Network Rail.
Work to improve track quality on the Brighton Main Line will affect some train times and some overnight services will be diverted to London Bridge, however the line will remain open.
The majority of the works are being carried out in South London.
In Sussex engineering work is taking place between Billingshurst and Littlehampton/Ford on Sunday May 27, closing various lines after 10.15pm.
The 21:33 and 22:35 London Victoria to Bognor Regis services will be replaced by buses from Billingshurst to Bognor Regis.
On the same day engineering work is taking place between Polegate/Pevensey Bay and Eastbourne, closing all lines until 8.35am and after 10.40pm.
Before 8.35am and after 10.40pm, buses will replace trains between Polegate and Eastbourne/Hastings/Ore.
On Sunday May 27 and Monday May 28 work is taking place between South Croydon and East Grinstead / Uckfield, closing all lines.
As a result, services between London Victoria and East Grinstead and between Oxted and Uckfield will not run.
Customers are advised to use alternative Southern services to Purley to connect with replacement buses to East Grinstead and Edenbridge Town. Buses will also run between Edenbridge Town and Uckfield.
John Halsall, Network Rail route managing director, said: “By doing these projects on a bank holiday we will affect fewer people than at any other time. This is especially true of South London, where our railway carries thousands of commuters every weekday.
“We do a great deal of maintenance work at night, which doesn’t affect trains, but the jobs we do at bank holidays can’t be done like that as they are too big and too disruptive. And of course, if we don’t do this kind of work, the railway will only get less reliable.
“So many thanks to all our passengers for their understanding and patience and keep checking before you travel.”