A post on Southern's Twitter page read: "The Brighton-bound line is blocked between Littlehampton and Worthing due to a fault on a train.
"Services to and from these stations may be cancelled or delayed by up to 15 mins."
Southern has confirmed all lines have re-opened and 'trains are beginning to return to normal', but warned they may still be cancelled or delayed 'whilst the service recovers'.
On its website, it added: "The fault in question has been confirmed as a dragging brake issue [on the train].
"Southern and Great Western Railway teams are working hard to get services back to the booked timetable.
"Disruption is expected until 9:30am."