Customers planning to take Great Northern trains from Hatfield to London are being asked to first go to Welwyn Garden City.
A fault with the signalling system between Welwyn Garden City and Potters Bar has caused disruption to trains running between these stations.
In response no trains are stopping at Hatfield station, in the direction towards London, to avoid a further build up of trains.
Great Northern passengers are advised to go from Hatfield to Welwyn Garden City and change there for a London bound service.
Those requiring a service from Welwyn Garden City to Hatfield have to now travel down to Potters Bar and change there to go back up to Hatfield.
Other Great Northern trains on the line may also be delayed by up to 15 minutes or cancelled.
The disruption is expected until 2pm.
For further updates please go to: greatnorthernrail.com/travel-information/plan-your-journey/service-updates
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