Major disruption on the rail network was the result of a "systemic failing" of the industry's timetabling system, GTR's outgoing boss has said.
Charles Horton told the Commons' Transport Select Committee on Monday that the new rail timetable was finalised just three weeks before it was implemented in May.
"Normally, where there is a major timetable change we would expect the timetable to be finalised at least 12 weeks in advance of its starting," he added.
Mr Horton, who announced his resignation on Friday, said the "biggest challenge" GTR faced in the lead up to the new timetable was timing.
The GTR boss was asked whether there was a single contributing factor on GTR's part as to why the timetable was "such a spectacular disaster".
He replied: "There is no single cause of what has happened here. This has been a systemic failure of the industry's timetabling system.
"The biggest challenge we faced is that normally, where there is a major timetable change, we would expect the timetable to be finalised at least 12 weeks in advance of its starting.
"On this occasion we were finalising the timetable three weeks before it started.
"And all the detailed complicated resource planning tasks that we would normally spread over a 12 week period, we had to complete in a three week period.
"And that caused us multiple problems in terms of the deployment of our drivers."
Mr Horton also said a decision by transport ministers to phase in the introduction of the timetable was one of the factors leading to ongoing delays and cancellations on the network.
He said that the process for approving the timetable became "extremely protracted and was complicated by a number of changes".
Mr Horton said: "One of those changes was there was a decision taken to phase in the timetable and that was made at the end of October 2017.
"That was a decision made by ministers."
Mr Horton stressed that the phasing of the new services was "not the only issue we were facing", noting that the "slow pace" of getting the timetables finalised contributed to GTR's difficulties.
Hundreds of services have been disrupted since schedules were changed on May 20.
Passengers using GTR and Northern have been particularly affected, with some stranded on platforms for several hours.
In the first two weeks, the proportion of trains either cancelled or delayed by more than 30 minutes was 13 per cent for GTR and 11 per cent for Northern.
Both operators introduced temporary timetables on June 4, removing around 6 per cent of daily services in a bid to boost reliability.
Northern's managing director David Brown said the operator was "on track" to deliver the new timetable until it was revealed in January that a Network Rail project to electrify the line between Manchester and Preston suffered a delay.
Mr Brown told MPs that this meant the timetable needed to be planned in just 16 weeks rather than the typical period of 40 weeks.
In conversations with other train operators and Network Rail, Northern asked for the national timetable change to be postponed so the previous schedule would continue.
"A significant number of other players didn't want that to happen and that option was not the preferred option," Mr Brown told the committee.
Additional reporting by Press Association.