AT&T is not battling with just the F.C.C. on this matter. Last fall, the Federal Trade Commission sued AT&T for the same practice, saying that the company had engaged in deceptive advertising. That case is pending.

The F.C.C. said on Wednesday that since 2011 it had received thousands of complaints from AT&T customers enrolled in unlimited data plans and that the service of millions of customers had been impaired. While AT&T no longer offers unlimited plans to new customers, it has allowed already-enrolled customers to renew their contracts.

“I entered into a contract believing I was paying for unlimited data at 4G speeds,” one complaint filed with the F.C.C. last October read. “Providing me less than that seems disingenuous at the least.”

The 2010 rule that the F.C.C. says AT&T violated is part of the agency’s 2010 Open Internet order, an earlier attempt by the agency to protect the principles of net neutrality, or the concept that no service provider should be allowed to manipulate a web user’s access to content. While most of that order was invalidated by a federal court after Verizon sued the F.C.C., the transparency rule applied in this case was upheld by the court and has been in effect since 2011.

Critics of the enforcement action said it highlighted that the F.C.C. had too broad and too subjective a say over what kind of behavior was acceptable from service providers, a criticism of the regulations approved this year. The agency’s Republican minority on Wednesday called the proposed $100 million penalty too severe.

“Here we are imposing a rigid standard that is based on a subjective opinion of what notification, in hindsight, should have been provided,” said Michael O’Rielly, a Republican commissioner.

According to senior F.C.C. officials, the fine could be reduced during the administrative process ahead and the agency will take into account what kind of flexibility AT&T might offer current subscribers to opt out of unlimited data contracts they have been locked into.