A signalling problem at Herne Hill station is causing some Southeastern Lines to be blocked.
Train services running through this station may be delayed or diverted between London Victoria and Bromley South.
Network Rail engineers are investigating the problem, but the disruption is expected until 9.30am.
For passengers who use the London Victoria to Orpington route, the Orpington bound trains will be diverted via Catford and unable to top at stations between Brixton and Beckenham Junction.
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