No mainline train services are able to enter or leave London's Euston station due to a fatality on the tracks in Hertfordshire.
National Rail Enquiries said the incident is affecting passengers travelling with Avanti West Coast, London Northwestern Railway and Southern. Disruption is expected to continue until 10am.
Avanti West Coast runs long-distance services between Euston and Glasgow on the West Coast Main Line, with branches to locations such as Birmingham, Manchester, Liverpool and North Wales.
British Transport Police said it received a report of a casualty on tracks near Watford Junction station, Hertfordshire at 6.