Delays are expected between London Kings Cross and Stevenage until at least 6pm due to a trespasser on the line.
The route was closed while emergency services attended the scene between Welwyn Garden City and Stevenage.
The trespasser has now been safely removed and lines have reopened, however train services scheduled to run through the area may still be cancelled, delayed by up to 40 minutes or revised as services recover.
Great Northern customers are advise to check before travelling and allow extra time to complete their journeys.
Tickets will be valid to use on the following alternative services:
• London Underground on all reasonable routes
• London Buses on all reasonable routes between London, Crewes Hill and Potters Bar
• Greater Anglia services between Cambridge, London and Hertford East
• Cross Country between Ely and Cambridge
• Metroline route 84 between New Barnet, Potters Bar and St Albans City
• Uno bus routes 601 between Welwyn Garden City and St Albans City, 602 between Hatfield and St Albans City, and 653 between Welwyn Garden City, Hatfield and St Albans City
•Arriva Buses route 724 between Hertford North, Welwyn Garden City, Hatfield and St Albans City
Mutual ticket acceptance is in place between Great Northern and Thameslink.
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