Thameslink warned passengers to check before travel between December 25 to 29 as track replacement works take place.
Track replacement on the Thameslink route in central London means that the line from St. Pancras International to Herne Hill is closed.
During that time, Network Rail engineers will install 1,670m of new rails on the track between Farringdon and Blackfriars.
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Trains between Bedford and Brighton will only run between London Bridge and Brighton, with two trains running an hour.
Services to and from airports will be affected during this period with fewer trains serving Gatwick Airport and Luton Airport Parkway.
Rail replacement buses will be operating on some routes, but these will be limited, and they are expected to be extremely busy.
Lucy McAuliffe, Network Rail’s Sussex route director, said: “The rails between St Pancras International and Blackfriars were installed in 2011, and after 13 years and thousands of trains passing over the track, the rails are worn and need replacing with brand new rail to prevent faults delaying passenger journeys.
“With the railway closed anyway on Christmas Day and Boxing Day, engineers will get a head start on the work ready for this crucial route to reopen.”
