There are delays on trains in and out of London Victoria due to an earlier trespassing incident between East Croydon and Clapham Junction.
Trains may be delayed by up to 15 minutes, cancelled or revised. The disruption is expected to continue until 12pm.
National Rail said: "All lines at Norbury have reopened after we confirmed that the trespasser had left the railway.
"This is part of the Brighton main line which is a busy part of the network and queues quickly form around the problem."
The problems at Victoria follow delays across the Southern rail network earlier this morning.