AT&T Thursday confirmed that it will begin throttling customers with 3G unlimited plans that exceed 3 GB of data in a single billing cycle. Speeds will return to normal at the start of the next billing cycle.

The change will also affect customers with a 4G LTE smartphone who still have unlimited plans. Data speeds for those customers will be reduced if usage is 5 GB or more in a billing cycle, and speeds will return to normal at the start of the next billing cycle.

The changes come amid reports that some unlimited customers had begun receiving text messages alerting them that they will be throttled for using too much data. Until now, those reports appeared to have been limited to users that fell into the top 5 percent of data users in particularly saturated markets.

An AT&T spokeswoman said today's announcement will extend the policy to all users of unlimited data plans, regardless of market. AT&T did not give a specific speed to which those who exceed the 3 GB limit will be throttled.

AT&T cancelled its unlimited plans last year but had allowed those who still had them to keep them. The company now offers 2 tiered plans: 3 GB for $30 and 300 MB for $20.

The company blamed skyrocketing data usage and scarce spectrum availability as the reason it had to make the change.

"How we're managing the network only affects a small minority of the heaviest smartphone data users still on unlimited plans," AT&T said. "Put another way, this does not impact more than 95 percent of our smartphone customers."

AT&T said customers will get a text message from the company before experiencing a change in speed, and it promises that even with reduced data speeds, customers will still be able to email and surf the Web and continue to use an unlimited amount of data each month.

Customers on tiered plans will not be affected, aside from the current 5 cents per MB of overage.

AT&T said it encourages all of its customers to use Wi-Fi whenever possible, especially when watching video, which it views as the most data-intensive activity.