Trains between Brighton and Haywards Heath are currently being delayed or cancelled due to a fault with the signalling system at Preston park, according to Southern Rail.
A Southern spokesman said train services running to and from these stations will be cancelled or delayed by up to 20 minutes due to the problem, identified by Network rail.
It added: "Disruption is expected until 20:00. Tickets will be accepted on Brighton and Hove buses.
"Valid tickets are permitted to travel with the following operators at no additional cost; Brighton & Hove buses between Brighton and Shoreham-by-Sea; Southern and Thameslink via any reasonable route to and from Brighton.
"If you're travelling into London this evening, an alternative route is via Lewes."
Let us know if you have been affected.