Avanti West Coast has been ordered to stop releasing tickets just a few days ahead of travel or face "formal measures".
The train operator has been warned by rail regulator the Office of Rail and Road (ORR) to submit an improved recovery plan for producing timetables by February 2.
Avanti West Coast runs trains on the West Coast Main Line between London Euston and Glasgow Central with branches to Birmingham, North Wales, Liverpool, Manchester and Edinburgh.