AT&T is imposing another $5-per-month price increase on customers who have held onto a decade-old grandfathered unlimited data plan. The new price will be $45 a month.

The data plan's price was $30 monthly for seven years until AT&T raised it to $35 in February 2016. A second $5 bump brought the price to $40 a month in January 2017. The third $5 increase in three years will kick in next month.

"Customers who have a grandfathered $40 data plan will receive notifications of a $5 per month rate increase for the data plan," AT&T said in the price increase announcement. "The rate increase will take effect starting with the customer’s July, 2018 service."

The $45 monthly fee is for wireless data only. These customers pay additional fees for phone calling and texting, roughly doubling the overall price.

AT&T urges switch to newer plans

AT&T could force these customers to move to newer plans because their contracts ran out years ago. Instead, the carrier has been implementing yearly price hikes and encouraging users to change plans. AT&T urged the customers to "learn more about the benefits of our currently available unlimited rate plans," while noting that customers who switch to a different plan "will not be able to switch back to their current grandfathered unlimited data plan in the future."

AT&T's current unlimited smartphone service starts at $65 a month for a one-line plan that caps video streaming speeds at 1.5Mbps and doesn't allow mobile hotspot usage. An AT&T unlimited plan starting at $80 allows high-definition video streaming and 15GB of mobile hotspot usage a month. Both provide access to HBO and $15 discounts on DirecTV service.

AT&T claimed that it is imposing the latest price increase simply to "make sure we continue to provide the best service for all of our customers," because the use of mobile data is hitting "record levels."

Customers on AT&T "unlimited" plans aren't charged extra for exceeding data caps, but they can be slowed down if they use more than 22GB in a single month. The slowdowns apply only in congested network areas, AT&T says.

"This price increase will not impact customers’ current unlimited data speeds," AT&T said.

The throttling policy used to be much more strict, and AT&T is still facing a lawsuit from the Federal Trade Commission over the practice.