First Student, which runs yellow school bus services, is expected to have suffered a 1.8% fall in annual revenues amid unprecedented school closures during a period of abnormal weather conditions across America, including widespread snow storms.
Its Greyhound coach business also looks set to see full-year revenues fall by 2.9% after the severe weather - which swept across North America from December and continued into March - caused significant disruption.
FirstGroup said overall operating profit for the year to the end of March was expected to be in line with forecasts, excluding the £14 million impact of the US weather.
The company had already warned in January that the weather would result in First Student seeing slower-than-expected progress towards profit margin targets in its turnaround plan.
Full-year results will be published on May 21.