London Midland says a train has derailed near Watford Junction causing major disruption to services in and out of London Euston station.
Virgin Trains says the London Midland service "hit a landslide causing it to strike another train and derail".
London Midland Milton Keynes to Euston train derails just outside Watford following flooding.No injuries.
A train has derailed near Watford Junction,operator London Midland has confirmed.
No-one was injured when the 06:19 BST service from Milton Keynes to Euston left the track.
The accident was caused by a reported landslide, believed to have happened after flooding in the area.
A spokesman said London Midland and Virgin services would be "severely disrupted" from the north-west of England, Scotland, and the Midlands.