Train operator GWR is having to cancel some services because of a “rising number of staff” with Covid-19.
The firm said it has “taken steps” to “maintain service levels across the network” but includes planned cancellations.
A statement said it was aiming to “minimise the impact on as many customers as possible” but added some services may be busier than usual.
Customers with booked tickets can travel on earlier or later services.
GWR serves stations from London to the West, South West and Wales.
Covid and strikes have affected a number of train services over Christmas including CrossCountry.
Southern has also announced none of its trains will run to or from London Victoria until 10 January due to “coronavirus isolation and sickness”.
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