Trains between London and Manchester and in the opposite direction have been cancelled after a person was hit by a train.
Emergency services are currently responding to the incident.
Avanti West Coast, which took over the running of the West Coast Mainline from Virgin last December, has warned services may be cancelled, delayed by up to two hours or revised.
It happened between Watford Junction and Milton Keynes Central.
All lines are said to be blocked.
Disruption is expected until at least 2.30pm.
Avanti West Coast said: "Cancellations to services between Watford Junction and Milton Keynes Central.
"Due to a person being hit by a train between Watford Junction and Milton Keynes Central all lines are blocked.
"Train services running through these stations may be cancelled, delayed by up to 120 minutes or revised."
Transpennine Express are conveying passengers between Sheffield and Stockport in both directions until further notice.
Northern are conveying passengers between Manchester Piccadilly and Leeds in both directions until further notice, Avanti West Coast said.
The operator said the following trains have been cancelled:
11:40 London Euston to Manchester Piccadilly, due 13:46
11:55 Manchester Piccadilly to London Euston, due 14:06
12:20 London Euston to Manchester Piccadilly, due 14:35
12:40 London Euston to Manchester Piccadilly, due 14:46
12:55 Manchester Piccadilly to London Euston, due 15:07
No further details have been revealed.
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