The RMT union has informed Southern Railway that its conductor members will strike on Wednesday May 18.
In case of strike, Southern services across Sussex will be affected, with no service on many routes and only a limited service between approximately 7.30am and 6pm, Southern Railway said.
Southern said it will extend the service hours if able to, but the last connecting train for commuters travelling from London to stations between Crawley and Chichester, Hove and Bognor Regis, or between Brighton and Eastbourne, may be as early as 4.30pm.
Queuing systems will be in place at many stations, but the railway company said they could not guarantee to get everyone to their destination.
Cycles will not be permitted on Southern, Gatwick Express or Thameslink trains (south of Blackfriars) on the affected date.
Gatwick Express and Thameslink services will be largely unaffected by the strike but some services are expected to be extremely busy on these days.