First Capital Connect has apologised to passengers for the severe disruption to the Thameslink route, advising them to allow extra time to complete their journey.
There is a reduced service with trains only running between Bedford and Luton Airport Parkway, Brighton and London Bridge, as well as the Wimbledon loop and London Blackfriars.
Tickets can be used on London Underground and London Buses as well as UNO and ARRIVA buses between Hatfield and St Albans.
Customers can also use London Midland services between London Euston and St Albans Abbey; and London Midland services between Euston and Bedford stations - using their existing travel ticket/card.
Great Northern services are running as planned and passengers can use their Thameslink tickets on this route.