Services are experiencing delays as one of the two railway lines between Brighton and London is currently blocked.
Services may be subject to delays of up to 10 minutes and platform alteration, a Southern Rail spokesman said.
According to the spokesman: "Train crew are investigating a fire alarm activation on board the 09:03 Brighton to London Victoria service.
"Until investigations are complete, trains within the area will continue to experience delays and short notice amendments."
Overall journey time towards London Victoria may be delayed by up to 20 minutes.
The disruption is expected to last until 11.30am.
Find out more on Southern Rail's website here.
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