An earlier fault with the signalling system is causing major delays between Rugby and London today (Thursday, July 13).
London Midland and Virgin Trains may be affected by up to an hour after the fault between Kings Langley and Milton Keynes Central at around 1pm.
All stations between Watford Junction and Milton Keynes Central were not served as the line was blocked.
It has been fixed but disruption is expected until the end of the day.
To follow this incident on Twitter, use #KingsLangley.