A Southern Rail spokesman said the disruption is expected to last until 7pm.
The spokesman said: “Due to a safety inspection of the track at Littlehampton trains will be using alternative platforms in and out of the station whilst Network Rail investigate further.
“Services running to and from this station may be delayed.”
Platform 1 at the station is currently closed.
The spokesman said: “A part of the track has been reported as defective at Littlehampton, until Network Rail operatives carry out their inspections, trains will be using alternative platforms in and out of Littlehampton.”
Services travelling through Gatwick Airport are also being delayed due to a shortage of train crew, a spokesman said.
No direct trains are running between Brighton and Three Bridges this week due to major engineering works.
Instead there are two trains running per hour from Brighton to London Victoria via Littlehampton, with extended journey times – read more here.