GWR said it “expects significant disruption to services this Sunday because of crew availability”.

Many staff are not contractually obliged to work on Sundays.

A reduced timetable will affect services between London Paddington and Bristol Temple Meads; London Paddington and South Wales; and London Paddington and Devon and Cornwall tomorrow (December 15).

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Fewer trains will run, they will be extremely busy and additional cancellations or delays may occur, said GWR.

“Customers are advised to change their plans and avoid travel unless absolutely necessary,” it added. 

GWR said visitors to Bath’s Christmas market on Sunday should not take the train.

Journey planners will be fully updated tomorrow morning.

Tickets for Sunday will be valid for Monday, December 16 or can be fully refunded.

It comes ahead of major disruption which will start over Christmas to create a new station for HS2 at Old Oak Common in west London.

From 27 to 30 December, most GWR services will terminate and start from Reading with passengers changing to the Elizabeth Line for a lengthy diversion to London Euston.