"Severe disruption" will impact Thameslink train services this weekend, with cancellations and reduced services expected.
On Saturday, December 30, customers have been warned to expect cancellations and have been asked to check their journeys before they travel.
On Sunday, December 31, trains will run during the morning but "a significantly reduced service" will be in place north of London and south to Brighton in the afternoon and evening.
Govia Thameslink Railway's customer service director Jenny Saunders said: "We’re very sorry that we’ve had to alter and reduce our service this weekend.
"As with train companies in other parts of the country, this is because of train crew shortages, with winter illnesses having a particular impact.
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“We’re doing our best to limit the impact and spread services but, regrettably, we strongly advise passengers to check their journeys before setting out.
"On Sunday, the service will be particularly affected in the afternoon with no Thameslink trains overnight.”
Ticket acceptance is currently being arranged for other modes of transport.
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