Trains may be cancelled or delayed by up to 30 minutes between London Blackfriars and London St Pancras International.
Thameslink trains could be disrupted until 5pm today (July 10) – over eight hours after issues began around 8.40am.
National Rail has warned: “An earlier signalling fault at City Thameslink has been severely disrupting northbound trains through London.
“In reaction, some southbound trains have also been affected on their return journeys.
“Congestion has reduced, but some trains have already been delayed by up to 30 minutes in the area.
“Thameslink have also had to make some changes to services, in order to prevent the delays worsening.
“This means your journey could still take much longer than usual.”
