Trains between Haywards Heath and Gatwick Airport are currently experiencing disruption due to a tree on the tracks.
According to National Rail, trains are having to run at reduced speed on all lines.
Train services running through these stations may be cancelled or delayed by up to 10 minutes and disruption is expected until 5pm.
National Rail said arlier today the 11:48 Brighton to Victoria service struck a tree in the Balcombe area, which caused some damage to the train before it was taken out of service at Three Bridges.
Services between Haywards Heath and Three Bridges are now running at a slower speed while assessments are made regarding the tree, National Rail added.