ITV News has learnt that Govia Thameslink (GTR) underestimated the scale of disruption caused by changes to rail timetables by ten fold.
In a presentation given to the Transport Secretary just three weeks before the new timetables started in May, the company suggested there were just 43 planned cancellations on its routes.
On the first working day of the new arrangements, the actual figure was 423.
Documents obtained by ITV News also show GTR bosses were confident of their own preparations, claiming that their plans were at "an advanced stage of preparation".
They flagged that there was an increased risk of further cancellations due to problems training drivers on new routes, but insisted that the “issues are well understood” and "communication of the changes is extensive".
Hundreds of thousands of passengers suffered chaos on GTR's lines during the first few weeks of the changes, with some problems continuing this week despite the introduction of a third new timetable.
Acute disruption has also been experienced on Northern Rail.
On Tuesday morning, commuter Emily Ketchin told ITV News that she no longer gets to see her children during the week because of the cancellations and delays to her route from Harpenden to London.
"I definitely don't get to see them [her children] in the evening, and unless they wake up early in the morning I don't get to see them," Ms Ketchin said.
"So I didn't see them yesterday and I won't see them today.
"Under the old timetable I did. All that's gone."
The train companies have subsequently admitted that they did not foresee the scale of the problems until three days before the timetables were due to be introduced, due to problems with driver rosters.
They insist they did signpost their concerns to the Government.
ITV News understands the unearthed documents could form part of an internal investigation at the Department for Transport into the handling of the chaos.
The Government has said it could strip GTR of its franchise if it can be proven that it breached its contract or acted with negligence.
A spokesperson for GTR said: “The scale of cancellations associated with the implementation of the new timetable was not – and could not have been - known at the time of this briefing.
“However, GTR confirmed there was a risk of cancellations due to the very late finalisation of the timetable by Network Rail and the impact this had on driver work schedules and rosters.”
A spokesperson for the Department for Transport said: “We have long been clear that while some minor risks of the new timetable were raised, there were assurances from industry that these would be mitigated before the timetable change on May 20.
"This presentation does not warn of the extent of the disruption that passengers would encounter and this was not raised by GTR in the meeting with the Secretary of State on May 2.
“An independent inquiry is being carried out to determine how this went wrong and to make sure this does not happen again.
"We are also conducting our own investigation looking at whether GTR has breached its contract and we won’t hesitate to take tough action against the operator if it is found to have been at fault.”