Trains from Gatwick Airport through to Brighton are currently facing delays of up to half an hour after a passenger fell ill.
Southern say disruption is likely until around 11am.
“The train crew on board the 9.16am London Victoria to Littlehampton service are unable to proceed beyond platform 7 at Gatwick Airport, as a passenger requires urgent medical assistance.
“Depending on the nature of the passenger illness, the situation will be dealt with appropriately.
“Often the emergency services will advise staff to leave the passenger on board the train, therefore queues can build up on the affected lines.
“Once it is safe to do so, the ill passenger will be assisted of the train and will receive the care they need once safely away from the platform.”
Currently platform 7 at Gatwick Airport is blocked but, with this in mind, a regular service will still run.
Trains may experience brief delays while they wait for the next available platform.