This will affect Great Northern and Thameslink trains from Monday. Picture: Casey Gutteridge. (Image: Archant)
Due to a signalling fault at Welwyn North, Great Northern is experiencing delays from Stevenage to Alexandra Palace.
Trains will also not be stopping at Welwyn North and Knebworth when coming from London.
Passengers will need to take a train to Stevenage and change for a London-bound service back to the station.
Disruption is expected to continue until 6pm.
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