A federal regulator criticized Wells Fargo for engaging in unfair and deceptive practices and failing to manage risks, and said it had not set aside enough money to pay back the customers it harmed.

The confidential report, prepared by the Office of the Comptroller of the Currency and reviewed by The New York Times, criticizes Wells Fargo for forcing hundreds of thousands of borrowers to buy unneeded auto insurance when they took out a car loan, as well as its handling of the problems once they were detected.

The regulators’ report, sent to the bank this week, is preliminary. Still, it represents the latest blow to the reputation of Wells Fargo, America’s third-largest bank and one that was once regarded as being among the best run in the country. The bank is still trying to recover from a scandal in which its employees created millions of credit card and bank accounts that customers had not requested, eventually leading to the ouster of the bank’s chief executive and millions of dollars in regulatory fines.

The comptroller’s findings could have a significant impact on the bank. The report stated that Wells Fargo had most likely underestimated how much it would cost to reimburse harmed customers. And it could force the bank to curb, or at least more closely monitor, its practices across the entire company.