Due to a fault with the signalling system between London Euston and Watford Junction some lines are blocked.
A signalling fault is currently causing disruption to train services from Milton Keynes Central.
Train services running to and from London Euston and Watford Junction stations are being cancelled, delayed by up to 60 minutes or terminated at and started back from Milton Keynes Central.
Disruption is due to be in place until 2pm today (23 January).