A signalling fault in the Embankment area is causing disruption to Thameslink trains through central London.
National Rail is warning trains between London Bridge and Blackfriars may be cancelled, delayed by up to 30 minutes or revised.
Disruption is expected until at 3pm, it says.
An earlier fault with the on-train signalling system has also delayed some southbound services between London St Pancras International and Farringdon.
