Commuters have been warned to expect severe delays on trains travelling between Birmingham New Street and London Euston.
Services running in and out of London Euston have been suspended since noon due to overheard wire problems in the Berkhamsted area.
Network Rail has closed all lines in the area to allow repairs to be carried out and delays for passengers travelling on Virgin Trains and London Midland services could continue into the evening.
A spokesman for Virgin Trains said: “Network Rail is working on the problem, but we do not know when things will be back up and running.
“Passengers should be aware that they may subjected to severe delays for the rest of the day and possibly into the evening.”
The spokesman added that existing tickets will be valid on alternative operators who do not travel into London Euston.
Chiltern Railway services, which operate between Birmingham Moor Street and London Marylebone are unaffected by the delays.
For more information visit www.nationalrail.co.uk.