The train company has received reports of a trespasser on the railway and, as a safety precaution, the electricity supply will be switched off.
"This means trains are currently unable to run between these stations," a spokesperson wrote on Twitter.
"The police are in attendance to bring the trespasser to a place of safety as soon as possible. In the meantime, to help you complete your journey, your ticket is being accepted on:@BrightonHoveBus between Brighton and Shoreham-by-Sea"
Trains between these stations will be delayed until approximately 10am.
Network Rail is also carrying out planned engineering works this weekend on parts of the network to maintain and improve the railway. Buses will replace trains between Three Bridges and Brighton / Lewes.
Two crashes have been reported on Sussex roads as of 9.30am.
The A29 near Slindon in West Sussex is partially blocked following a single-vehicle accident.
The car reportedly left the road and paramedics are at the scene.
Minor delays have been reported on A29 Fairmile Bottom both ways from A284 London Road (Whiteways Lodge roundabout) to Madehurst turn off.
In Isfield, near Uckfield, Bradfords Lane is partially blocked following a collision between two vehicles. Traffic is said to be coping well.
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