Rail passengers are facing cancellations and delays due to a signalling fault affecting Thameslink services.
Network Rail said problems on the line between Luton and London St Pancras International blocked services in both directions.
Thameslink said although the fault has been fixed, services would be disrupted until at least 2pm today (Friday), then later extended it until 4pm.
"As trains return to normal some services will be delayed by up to 60 minutes, cancelled or revised due to trains and train crew out of position," it said.
Thameslink trains between Brighton and Bedford, which call at London Gatwick, are also affected.
Services between London St Pancras and Nottingham, Leicester, Derby and Sheffield are also affected.
