Trains between London and the South coast are being affected after an electrical fault at Balcombe this morning.
Network Rail say engineers have now resolved the fault but residual delays will occur while the services recovers.
Train companies with services affected include Southern and First Capital Connect.
Services affected include train routes to Lewes, Hove, Brighton, Preston Park, Burgess Hill, Haywards Heath, Balcombe, Three Bridges, Gatwick Airport, Purley, East Croydon, Herne Hill, Elephant & Castle, London Blackfriars, London St Pancras International, Luton & Bedford, Clapham Junction and London Victoria.
Network Rail said the electrical fault had caused signalling problems at Balcombe.
It said trains between Brighton and Lewes and Gatwick Airport may be delayed by up to 30 minutes. These delays will continue until further notice.
To find out whether this will impact your journey and to get alternative options planned for you, use the National Rail Enquiries real-time Journey Planner, or call TrainTracker on 0871 200 49 50.