All Thameslink lines are running at reduced speed thanks to the disruption.
Thameslink trains tweeted at 6.56pm: “Due to trespassers on the railway at London Bridge trains have to run at reduced speed on all line[s]”.
Some lines in and out of London are completely blocked, while “other services are able to move at caution into and out of the station, but this has reduced capacity at one of the busiest points of the network”, according to the operator.
The delays could affect passengers at St Albans, Harpenden and Radlett.
Trains may be delayed by up to 30 minutes, or even cancelled or revised.
The disruption can be expected until 8.30pm.
Southern Rail will accept Thameslink tickets, and London Underground can be used between London Bridge, London Blackfriars and London Victoria.
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