Thameslink and East Midlands Trains services are subject to severe delays this afternoon, after wiring damage caused yesterday led to signalling problems that have persisted today.
National Rail said a severely limited train service will be in operation and customers are advised to use alternative routes, as services that are able to run are likely to be extremely busy.
The rail operator expects this disruption to continue until the end of service.
Thameslink services between Bedford/Luton and Three Bridges/Sutton/Sevenoaks/Gatwick Airport/Brighton and East Midlands Trains (EMT) services between Corby/Nottingham/Derby/Sheffield and London St Pancras International have been affected by signalling problems.
Thameslink trains may be cancelled or delayed by up to 90 minutes, National Rail said, and EMT trains that are able to run are expected to be delayed by up to 60 minutes.
"On Tuesday afternoon, damage was caused to the overhead wires between Luton and London St Pancras International," National Rail said in a statement today.
"Network Rail were able to fix the wires, but a signalling problem occurred as a result of this damage. This is causing major disruption to journeys between Bedford and London St Pancras International.
"A severely limited train service will be in operation and customers are advised to use alternative routes, as services that are able to run are likely to be extremely busy. We expect this disruption to continue until the end of service"
EMT customers have been advised that a special timetable is in operation, but have also been warned that because there is a higher demand for trains that are running today, they are likely to be very busy.
Thameslink customers have been advised that, where possible, they should travel on Great Northern services.