AT&T announced service throttling for the heaviest users of its unlimited mobile data plans on Friday, saying the only way to truly solve its bandwidth crunch is for regulators to approve its merger with T-Mobile USA.

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, AT&T said in a statement on its website. 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 announced the US$39 billion merger plan in March, 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.

The throttling is intended to create a better service experience for all subscribers, AT&T said. The carrier had 98.6 million wireless subscribers at the end of the second quarter, including users of simpler feature phones as well as data-oriented smartphones. It said 15 million of its smartphone subscribers are on tiered data plans, which won't be affected by the new policy.

AT&T discontinued its unlimited data plans last June, before the introduction of the iPhone 4, but allowed subscribers who already had those plans to keep them. New subscribers got a selection of tiered plans to choose from, with limits on the amount of data they could download each month.

AT&T isn't the only carrier reconfiguring its data plans. Earlier this month, Verizon Wireless also discontinued its unlimited plans, and in April, T-Mobile announced it would throttle the bandwidth of users on unlimited plans after they used 2G bytes in a month.

Stephen Lawson covers mobile, storage and networking technologies for The IDG News Service. Follow Stephen on Twitter at @sdlawsonmedia. Stephen's e-mail address is stephen_lawson@idg.com