FirstBus has apologised "unreservedly" to users in the Forth Valley after widespread disruption to services, the BBC Scotland news website has learned.
Customers have experienced numerous delays and cancellations
Unannounced cancellations or delays to numerous routes across the area have been reported after an unknown number of buses were taken off the road.
FirstGroup insisted the vehicles were not taken out of service because they were unfit.
The Traffic Commissioner for Scotland has issued a warning to the company.
Joan Aitken said there was a "legal duty" on First to run its services according to the registered timetable.
Officials from Stirling Council met with First on Wednesday following complaints from members of the public about problems on some scheduled routes.
During the meeting FirstBus advised that it could be up to three weeks before normal services resumed.
On its website, the local authority said that "due to operational difficulties" FirstBus services were being disrupted.
Officials at Falkirk, Stirling and Clackmannanshire local authorities said the issue had not affected the provision of school buses contracted from FirstBus.
Ms Aitken said she had become aware of the issue in recent days and warned First that failure to comply with registered timetables could result in a financial penalty.
She added: "I know how important it is that buses turn up on time. We all rely on bus services to get us to our destinations safely and on time.
"Only factors out with First's control could excuse any failures to operate to time."
The company, which is the UK's largest bus operator and runs more than one-in-five local services nationally, carries an estimated 2.8 million passengers a day.
The commissioner said services must be maintained
A spokesperson for FirstGroup said the company "unreservedly apologised" to customers for the "temporary availability problems".
She added: "This has been due to an engineering review of our fleet to improve services in the longer-term.
"Concerns have been raised by a number of passengers and these are being dealt with on an individual basis.
"No vehicles have been withdrawn from operation on the basis that they are unfit for service.
"We have taken some vehicles out of use as a part of our strategy to improve reliability for customers in the longer-term and apologise for any inconvenience this has caused."