The signalling fault has now been repaired but delays are still expected.
Trains between Milton Keynes Central and Watford Junction have been affected.
The delays involve Avanti West Coast, London Northwestern and Southern services.
Trains also have to run at reduced speed on the line towards London Euston.
⚠️ A fault with the signalling system between Milton Keynes Central and Watford Junction is causing disruption to journeys through these stations. ⚠️ #WatfordJunction See Great Britain train incidents on your iOS homescreen with the Train Beacon widget pic.twitter.com/4FQfJBq1r0— Train Beacon Augmented Reality 🚄🚄🚄 (@TrainBeacon) November 18, 2021