Southern Rail and Thameslink said they are 'expecting a number of cancellations' across the network, as a result of train crew unavailability.
"You'll need to check journey planners before you set off for the station as these will be updated to show the cancelled services," a Southern spokesperson said.
"You'll need to plan ahead and allow extra time to complete your journey, if necessary."
There is also planned engineering work that may affect your journey.
Southern added: "Engineering work is also affecting some of our coastal, and South London 'Metro' services between London Bridge and Streatham."
Meanwhile, a short-notice change to the timetable between Polegate and Hampden Park means trains running between these stations will be cancelled.
Southern said suppliers have been unable to provide buses or drivers for some of the planned replacement bus services.
'Efforts have been made to get resources in place' but 'this has not been possible'. Disruption is expected until the end of the day.
"Unfortunately, Southern have had to make short-notice changes to the replacement bus timetable for this route today," a spokesperson said.
"The direct shuttle buses between Polegate and Hampden Park (and vice versa) will not be able to run. This will affect journeys throughout the day."
Buses are still expected to run on the following routes; between Eastbourne, Polegate and Pevensey & Westham and between Eastbourne and Hampden Park.
To travel between Polegate and Hampden Park, you will need to take a bus to Eastbourne and change there. You will also need to change at Eastbourne if you are making the journey in the opposite direction.
"Unfortunately, this will mean your journey will take around 30 minutes longer than usual," Southern said.
"If you are making a journey which connects with a train service from Polegate or Pevensey & Westham, you may wish to make your way directly to these stations instead, if you are able to."
Train times will change from tomorrow (Sunday, December 12) as Govia Thameslink Railway (GTR) starts to introduce an updated winter 2021-22 timetable. Click here to read more