Southern rail is urging passengers to check their journeys before travelling today, saying trains across the whole of its network may be cancelled or amended.
Services through stations including New Cross Gate, Anerley and Crystal Palace could be affected.
The train operator warned that there is a shortage of conductors and drivers due to sickness.
The company has already faced conductor strikes and is seeking an injunction to prevent a threatened strike by drivers in a dispute over driver-only trains.
The Rail, Maritime and Transport Union (RMT) has accused the company's owners of a "disgraceful campaign of misinformation" over the staffing "crisis" on Southern services.
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General secretary Mick Cash said: "The hard fact is that this company does not employ enough staff to fill the rosters and is carrying significant levels of vacancies.
"They do that because it saves them money and allows them to pump up the profits which can then be shipped across the Channel to subsidise their operations in France.
"For Southern/GTR to try and turn the blame for this staffing crisis onto the union is ludicrous when the public can see clearly that the fault lies squarely with this penny-pinching outfit who will stop at nothing to fleece this franchise, and its passengers, for every penny that they can."
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