Trains running between Seven Sisters/Tottenham Hale and Broxbourne are subject to severe disruption this afternoon due to an incident on the track.
Stansted Express services are experiencing delays and may be cancelled, with disruption expected to continue until 8pm.
"Emergency services are dealing with an incident between Seven Sisters / Tottenham Hale and Broxbourne. While they are dealing with the incident, all lines will be blocked between these stations," said National Rail.
"As a result, your train may be delayed by up to 60 minutes or revised. We anticipate a normal service will resume at 20:00."
The disruption comes as London's commuters grapple with ongoing industrial action by Southern rail workers.
This afternoon, talks between the Aslef union and rail bosses, aimed at bringing the Southern strikes to an end, broke down with no resolution achieved.