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Rail passengers are being warned to avoid travel on Christmas Eve when further strike action over pay and conditions is due to take place.
The planned walkout by RMT members on 24 December will have a severe impact on services, said Network Rail.
It advised passengers to travel only if "absolutely necessary" or consider travel on another day.
Most people should try to complete their journeys by 15:00, ahead of union members walking out at 18:00, it added.
The strike by RMT workers will continue until 06:00 on 27 December.
Network Rail added that some rail companies will be unable to run any services on Christmas Eve, with further "significant impacts" between Christmas and New Year.
Andrew Haines, chief executive of Network Rail said: "I am so sorry that our passengers are having to bear the brunt of the RMT's needless strike when a fair [pay] offer is on the table."
Previous strikes by the RMT have had a major impact on services throughout England, Scotland and Wales. Union members have rejected offers in a dispute over pay, job security and working conditions.
The RMT has announced further strikes for 3-4 January and 6-7 January.