Rail passengers have been warned of disruption on Monday morning as weekend engineering work overruns.
Work has been taking place on Brighton Mainline between Gatwick Airport, Redhill and Reigate, Purley and East Croydon.
National Rail said on Sunday the work was not expected to be finished in time, with disruption to services likely until the end of Monday.
Thameslink and Southern rail also warned of disruption and said only two of four lines between Gatwick and East Croydon would be available on Monday, which meant there would be “a reduced service across the network”.
Passengers were advised to check journey planners ahead of travelling and allow extra time to complete journeys.
National Rail provided a list of expected changes but said other short notice changes and delays were also likely.
Journey planners were due to be updated by 22:00 GMT on Sunday, it added.
People who want to delay their journey until Tuesday will be able to use Monday-dated tickets on Southern, Thameslink and Gatwick Express services, National Rail said.
Southern services between Reigate and London Victoria and between Redhill and Tonbridge in Kent will run normally.
Network Rail originally scheduled the work to take place over three weekends in January, with further dates planned in February, March and May.
Work was planned over winter to protect the line during peak tourism season, Network Rail said.