National Rail has confirmed that trains between London St Pancras International and London Blackfriars may be delayed by up to 40 minutes, cancelled or revised.
This is due to a fault with the signalling system between City Thameslink and London Blackfriars.
Some lines are disrupted and major disruption is expected until the end of the day.
Thameslink services are expected to be delayed by around 20-40 minutes when travelling southbound from City Thameslink towards London Blackfriars.
Thameslink has made a number of changes and cancellations to services and are working to put a plan in place according to National Rail.
Trains between Bedford and Three Bridges via Redhill will be suspended.
Peak-time services between Luton and Orpington will be suspended.
Peak-time services between Bedford and East Grinstead via London Bridge will be suspended.
Peak-time services on the Sevenoaks route will only run between London Blackfriars and Sevenoaks, with trains on this route not running between Welwyn Garden City and London Blackfriars.
Trains between Luton and Rainham will run as a limited shuttle service between Gravesend and Rainham, and will not run between Luton and Gravesend.
