Emergency services have been called to Hastings railway station to assist a passenger who has been taken ill.
The passenger became ill on board the 3.01pm Brighton to Ore service.
Services between Hastings and Ashford International have been disrupted.
Southern said the delays are expected until 7pm.
A spokesman said: “A passenger has taken ill on the 15:01 Brighton to Ore service. “Emergency services are on their way to Hastings to meet the passenger there.”
Passengers have been advised to check their journey before travelling.