A person has been hit by train between Welwyn Garden City and Finsbury Park, blocking all lines between Stevenage and London.
Great Northern said trains may be delayed by up to an hour, cancelled or diverted via Hertford North.
There is currently no service running in either direction from Hatfield and Potters Bar, while there is no service running between Welwyn Garden City and Moorgate.
Rail replacement travel is being organised between Welwyn Garden City and Alexandra Palace, with all tickets being accepted on the London Underground.
There is no estimated time when lines will re-open, but disruption is expected to last until at least 12.00pm.
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