Govia has ordered rail replacement buses for its Great Northern services at Welwyn Garden City, Hitchin and Cambridge stations.
The operator, which ordered the buses at 3pm today, said trains could be cancelled and an additional 30 to 60 minutes added to journeys.
The disruption will especially affect trains to and from Moorgate, including services on the Hertford North and Welwyn Garden City routes.
Great Northern said train crew unavailability was the reason for the disruption, and added that buses will not run with a specific schedule in mind.
"You should speak to a member of staff to find out more information, including the pick up & drop off locations," Great Northern said.
"These will also be shown on posters at most stations."
If you want more information on service updates, check out the Great Northern website.
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