Trains are not calling at Watton-on-Stone this morning due to a safety inspection of tracks between the station and Hertford North.
Services through these two stations on the Hertford loop - which starts at Stevenage - are set to be cancelled or delayed, and journey times will be extended by approximately 15 minutes.
Customers travelling from Watton-at-Stone are advised to take a bus service - which is currently in place - to Stevenage, and change there for services to London.
Disruption is expected until 9am.
Trains services across the Great Northern network are set to be affected today, due to unavailability of crew, and overall journey time may be extended by 30 minutes to one hour.
Rail operator Govia Thameslink has requested rail replacement buses to be based at Welwyn Garden City, Hertford North, Alexandra Palace, Hitchin and Cambridge this afternoon.
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