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Trains will be running under a reduced timetable due to high numbers of railway staff being forced to isolate due to Covid.
Southeastern confirmed "an increasing number of colleagues" have been affected in recent weeks meaning services across Kent will be cut back.
Passenger numbers have also fallen in response to the latest government advice on working from home.
Speaking at a Downing Street briefing earlier today, Prime Minister Boris Johnson acknowledged services would be disrupted and the days to come would be "challenging".
However, he did not bow to pressure from backbenchers, including Kent MP Craig Mackinlay, who had called on him to shorten the isolation period from seven to five days.
The train operator says 7% of its services will be reduced to make sure there is enough capacity and connections for passengers still relying on the rail network to travel.
It says the change to the timetable, which comes into effect on Monday, January 10, is designed to prevent cancellations to trains at short notice.
Passengers are warned to make sure they check before they travel.
Changes to the Southeastern network include:
David Wornham, passenger services director for Southeastern, said: “Like many other operational businesses, many of our frontline colleagues are being affected by the virus, and this in turn impacts the train services we can reliably operate for passengers.
"We’ve therefore decided to bring in a reduced timetable, which affects around 7% of our train services, so that we can offer passengers more certainty that trains will be operating and on time.
"We’ll keep the reduced timetable under constant review, alongside reacting to any updated travel advice and guidance when this is issued."
Full details of the reduced timetable will be updated to online journey planners, Southeastern's website and National Rail Enquiries from Friday.
Passengers are also encouraged to check how busy a train might be by using the Southeastern SeatFinder tool on the company's website.
Face coverings on trains are now compulsory again unless a passenger is medically exempt and fines may apply.