All trains between Milton Keynes and London Euston have been suspended.
An overnight lineside fire was reported near Harrow and all services to and from London Euston have been disrupted.
A spokesman from London Midland said in a tweet: "Services will start from and terminate at Northampton or Milton Keynes until further notice.
"It is likely that disruption will last all day."
Tickets with other train operators via any other reasonable routes will be accepted, London Midland Trains tweeted.
Any services travelling north from Milton Keynes and Northampton should not be affected.
London Midland is arranging road transport where possible.
The fire started at around 23:45pm yesterday at a timber yard in north-west London. A wall next to the track is now in danger of collapse.