Theft of signalling cables has caused “major disruption” on parts of the train network, National Rail said.
The cables were stolen at Napsbury, resulting in a fault with the signalling system at St Albans, Hertfordshire.
National Rail said East Midlands Railway services between Sheffield, Nottingham, Corby and London St Pancras were affected as well as Thameslink services between Bedford and East Croydon, and also between Luton and Rainham, in Kent.
A spokeswoman said work to replace the cables was taking place overnight and normal services were expected to resume at about 06:00 GMT on Sunday.
The company said the theft resulted in “major disruption” meaning that “trains running between Luton and London St Pancras International may be cancelled, severely delayed by up to 60 minutes or revised”.
On National Rail’s website, East Midlands Railway advised customers to expect delays as “trains are being manually sent through the affected area”.
Thameslink also told customers to expect lengthy delays and more frequent train change.
Alternative arrangements for travel have been posted on the National Rail site.
