An inquiry will be launched into whether there was a breach of contract after a timetable overhaul sparked major disruption on the rail network, Transport Secretary Chris Grayling has announced.
Mr Grayling apologised to rail passengers for the disruption they have faced and said he was "incredibly frustrated" by the timetabling problems.
A special compensation scheme will be put in place for Northern and Govia Thameslink Railway (GTR) passengers, he added.
The Transport Secretary told the Commons on Monday that the rail firms were not sufficiently prepared for the new schedule.
He said GTR "assured me personally that they were ready" just three weeks before the introduction of the new timetable.
The Office of Rail and Road will carry out the inquiry into the failure to implement the new timetable, he said in a statement to the Commons.
The final report will be published by the end of the year.
If GTR is found to be in breach of its conditions, Mr Grayling said he will consider enforcement action.
This means it could lose the right to bid for further rail franchises.
Northern will also be assessed over whether it was appropriately prepared.
Northern and GTR bosses will meet MPs in Parliament this week.
The Transport Secretary said he expects the new May timetable will now be introduced in stages over the coming months.
