Govia Thameslink Railway (GTR) has been fined a record £5m by the rail regulator over its chaotic introduction of a new timetable in May last year.
The Office of Rail and Road (ORR) said the rail company "failed to provide appropriate, accurate and timely information".
The new timetable caused severe disruption on Thameslink and Great Northern routes for eight weeks.
GTR has 21 days to respond to the penalty notice.
It is the first time the ORR has issued a fine for breach of licence.
The ORR said passengers were left with "little certainty" trains would run.
Some trains were permanently removed from the timetable but this was not made clear to passengers for several weeks, according to the ORR.
Other trains were removed or cancelled on a daily basis, leading to a "severe lack of certainty for passengers up until the point of travel", the regulator said.
Also, there was inadequate internal communication within GTR, which left station staff with "little or no information" to help passengers.
Stephanie Tobyn, deputy director, consumers, at the ORR, said: "The exceptional circumstances that followed the introduction of the timetable meant that providing perfect advance information for passengers was from the outset an impossible task and GTR's overriding focus was on providing as much capacity as it could to meet customer demand.
"However, persistent and prolonged failures in information provision meant that passengers couldn't benefit from the operational improvement it was trying to make."
A separate ORR investigation into Northern, which operates services across northern England, found in many cases passengers were given "inadequate information" in the two weeks that immediately followed the timetable introduction.
But the regulator concluded that the firm "subsequently took reasonable steps" to rectify this, meaning that no further action will be taken in relation to the issue.
The ORR has written to all train companies and Network Rail asking them to review their crisis management plans and ensure appropriate arrangements exist for helping passengers with disabilities in times of disruption.