Starting Oct. 1, heavy data users may see their downstream connections slow down after their usage reaches the point where they are in the top 5 percent of users in a given billing cycle, . The change only affects subscribers with unlimited data plans, leaving those with tiered plans free to buy more data if they run out.

In its statement, AT&T called the change one step to manage the rapidly growing demand for mobile data, saying it is also investing in its network and trying to acquire more network capacity.

"Many experts agree the country is facing a serious wireless spectrum crunch," the company said. However, throttling heavy users' bandwidth won't solve the capacity issue by itself, AT&T warned. "Nothing short of completing the T-Mobile merger will provide additional spectrum capacity to address these near-term challenges," the company said. AT&T , citing the need for a bigger pool of spectrum. The deal faces opposition from rivals and some lawmakers.

The carrier emphasized that the new policy will affect only a relatively small proportion of customers. "To rank among the top 5 percent, you have to use an extraordinary amount of data in a single billing period," the statement said. On average, those subscribers use 12 times more data than the average customer, according to AT&T.

Online gaming, heavy daily streaming of video and music, or sending large data files such as videos are among the uses that might lead to that point, it said. Data use on Wi-Fi won't count toward the total. AT&T promised to provide multiple notices and a grace period before it imposed throttling on a subscriber. The user's speed would go back to normal at the beginning of the next billing period.