The Federal Communications Commission today said it plans to fine AT&T $100 million for throttling the wireless Internet connections of customers with unlimited data plans without adequately notifying the customers about the reduced speeds.

"The Commission charges AT&T with violating the 2010 Open Internet Transparency Rule by falsely labeling these plans as 'unlimited' and by failing to sufficiently inform customers of the maximum speed they would receive under the Maximum Bit Rate policy," the announcement said.

The action isn't yet final. The FCC issued a Notice of Apparent Liability against AT&T that includes the proposed fine and provisions designed to bring AT&T into compliance with the commission's rules about making proper disclosures to customers.

AT&T can ask the commission to reduce or eliminate the fine, which would be deposited into the US Treasury. But even if AT&T opposes the fine, the commission says the company must file a report within 30 days detailing steps it is taking to correct misleading and inaccurate statements to consumers and to make more specific disclosures about data speeds. AT&T was ordered to notify unlimited data plan customers that its "disclosures were in violation of the Transparency Rule, and that AT&T is correcting, or has corrected, its violation of the rule with a revised disclosure statement."

AT&T was also ordered to let affected customers cancel their plans without penalty.

"Consumers deserve to get what they pay for,” FCC Chairman Tom Wheeler said in the announcement. “Broadband providers must be upfront and transparent about the services they provide. The FCC will not stand idly by while consumers are deceived by misleading marketing materials and insufficient disclosure.”

The Open Internet Transparency Rule is the only major portion of the 2010 net neutrality order that survived a court review. The FCC could have tried to penalize AT&T using the 2015 version of its net neutrality rules, but it chose the safer course of using a rule that has already been upheld. The FCC's latest rules are being challenged in court by broadband providers.

AT&T started offering unlimited data in 2007 when it partnered with Apple to sell the iPhone. The company no longer sells unlimited data to new customers but allows existing ones to keep the plans.

"Although the company no longer offers unlimited plans to new customers, it allows current unlimited customers to renew their plans and has sold millions of existing unlimited customers new... contracts for data plans that continue to be labeled as 'unlimited,'" the FCC said. "In 2011, AT&T implemented a 'Maximum Bit Rate' policy and capped the maximum data speeds for unlimited customers after they used a set amount of data within a billing cycle. The capped speeds were much slower than the normal network speeds AT&T advertised and significantly impaired the ability of AT&T customers to access the Internet or use data applications for the remainder of the billing cycle."

AT&T had been throttling all LTE customers with unlimited data plans once they hit 5GB of usage each month, but the company changed the policy last month so that the customers are now only throttled when the network is congested.

In addition to the FCC fine, AT&T is being sued by the Federal Trade Commission, which is seeking a court judgment that would bring millions of dollars of refunds to consumers.

The FCC said it believes millions of customers have been affected by AT&T's throttling, with speed reductions that "imped[ed] their ability to use common data applications such as GPS mapping or streaming video." On average, customers' speeds were slowed for 12 days per monthly billing cycle, the FCC said.

We interviewed one of those customers last December; speed tests showed the customer's cellular speeds were reduced from 23.51Mbps to just 0.11Mbps.

AT&T is gearing up for a fight. The company provided this statement: "We will vigorously dispute the FCC’s assertions. The FCC has specifically identified this practice as a legitimate and reasonable way to manage network resources for the benefit of all customers and has known for years that all of the major carriers use it. We have been fully transparent with our customers, providing notice in multiple ways and going well beyond the FCC’s disclosure requirements."

While it's true that other major carriers throttle unlimited data in certain instances, AT&T was alone among the biggest providers in throttling speeds even when its network wasn't congested.

The proposed fine was approved by Wheeler's Democratic majority, while Republican Commissioners Ajit Pai and Michael O'Rielly opposed the punishment of AT&T.

Pai said that "the Commission simply ignores many of the disclosures AT&T made" and that "instead of coming up with a common sense metric, the Commission threw a dart and came up with a $100 million forfeiture."

The FCC said its rules allow for a fine of $16,000 per violation, but actually imposing that amount "in this case would lead to an astronomical figure" because so many customers were affected.

The commission chose $100 million because, even though it "represents a small fraction" of the revenue AT&T has made from unlimited data plans, it is still large enough "to account for the gravity of AT&T's violations" and "deter future violations by imposing a penalty that will not be viewed by AT&T as a mere 'cost of doing business.'"

AT&T reported revenue of $32.6 billion and operating income of $5.5 billion in the most recent quarter. Wireless revenue accounted for $18.2 billion of the total, with a wireless operating income margin of 24.5 percent.