London Northwestern Railway passengers are facing disruption between Milton Keynes and London Euston due to signalling faults.
A fault with the signalling system between London Euston, Shepherds Bush and Watford Junction is causing major disruption to journeys between these stations.
London Northwestern Railway services between London Euston and Milton Keynes Central are currently affected.
Services may be cancelled, delayed by up to 90 minutes or revised.
National Rail anticipate major disruption will continue until 17:00.