The FTC is suing AT&T for misleading millions of customers on "unlimited" data plans by throttling speeds after as little as 2 gigabytes of usage.

The Federal Trade Commission announced Tuesday that it had filed a complaint in federal court against the telecommunications giant. The regulator claims that customers were not adequately informed that their service could be seriously restricted if they exceeded a certain amount of data in a particular billing cycle.

The FTC claims AT&T began throttling unlimited customers in 2011, lowering data speeds by as much as 90%. The regulator believes at least 3.5 million customers have been throttled. It also noted that AT&T had received thousands of complaints over its data speed.

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“AT&T promised its customers ‘unlimited’ data, and in many instances, it has failed to deliver on that promise,” said FTC Chairwoman Edith Ramirez in a press release. “The issue here is simple: ‘unlimited’ means unlimited.”

Ramirez said on a call with journalists that the FTC was seeking a financial settlement on behalf of AT&T customers, although she could not provide details on the possible size of a settlement.

There is some debate about how much data the average smartphone customer uses — Nielsen found 733 megabytes per month in 2013, while Cisco reported 529 MB per month.

This is well below the 2 gigabyte threshold the FTC found, although it is worth noting that Cisco also projects the average use to consumer 2.7 GB per month by 2018. Overall usage is projected to rise as the rollout of faster networks combines with greater demand for data-intensive media such as streaming video.

AT&T denied the allegations.

“The FTC’s allegations are baseless and have nothing to do with the substance of our network management program," Wayne Watts, AT&T general counsel, said in a statement. "It’s baffling as to why the FTC would choose to take this action against a company that, like all major wireless providers, manages its network resources to provide the best possible service to all customers, and does it in a way that is fully transparent and consistent with the law and our contracts."

Watts added that AT&T had disclosed in 2011 that the company planned to slow data speeds for customers using large amounts of data, and that throttling affected only about 3% of the company's customers.

Federal regulators have begun to pay closer attention to how telecom companies operate their networks in relation to their marketing. Tom Wheeler, Federal Communications Commission chairman, warned Verizon and several other wireless companies about slowing the speeds of customers who use large amounts of data on unlimited plans. Verizon backed down from plans to throttle certain users on its 4G network as part of its "network optimization," but it can still throttle 3G users.

Throttling itself is not illegal, and the FTC is not going after that practice, Ramirez said. Telecom carriers are not required to provide equal data speed to all customers. The issue that both the FTC and FCC are addressing is the misleading nature of the claims of "unlimited" plans in relation to data speed. Ramirez said that lawsuits against other telecom companies for similar infractions could be on the way, but declined to comment on any specific companies.

Ramierz added that the FTC found AT&T's throttling "had no particular relation to network congestion." FTC attorney Evan Rose also said that throttling affected a much larger percentage of AT&T's wireless customers than the company claimed.