A fault with a signalling system is causing disruption to train services between Manchester Piccadilly and London Euston.
Trains may be delayed by up to 90 minutes or cancelled, according to National Rail Enquiries.
An announcement on the website states: "A fault with the signalling system in the Cheddington area is resulting in disruption to trains between London Euston / Clapham Junction and Milton Keynes Central.
"Trains may be delayed by up to 90 minutes or cancelled until the end of the day."
Avanti West Coast services between Euston and Piccadilly are affected.
The train operator tweeted about the incident, saying: "Due to overhead wire problems between London Euston and Milton Keynes Central the line is blocked.
"Departures from London Euston have been suspended. Updates to follow."
The disruption is expected to last all day, according to a tweet by National Rail.
Passengers can use tickets on alternative services.
For more information, click here.
