By Thomas Gryta 

T-Mobile US Inc. is raising the price of its unlimited wireless data plan by $10 to $80 a month, as increased usage and an upgrade to faster wireless broadband increases the cost of providing that service.

The move is an unusual one for the fourth-largest U.S. carrier by customer base, which has reversed a long decline in its subscriber base by doing away with carrier standbys like service contracts and international data fees. Its low-priced unlimited data option--something Verizon Wireless and AT&T Inc. don't offer--has been a key to competing with the market's leaders.

The change will take hold later this month. T-Mobile is also building in more tiered data options, in which subscribers pay for fixed allotments of high-speed data that are cheaper than the unlimited plan. The company is adding a new tier with 5 gigabytes of data and boosting data allotments at other tiers. For example, a $50 monthly plan that now offers 500 megabytes of high-speed data will soon give subscribers a full gigabyte.

AT&T and Verizon Wireless stopped offering new customers unlimited data years ago in favor if tiered plans where subscribers would pay more for bigger allotments. With subscriber growth slowing, the moves were aimed at positioning the carriers to capitalize on growing wireless Internet use, particularly for video.

T-Mobile Chief Marketing Officer Mike Sievert said the move to raise prices for unlimited customers partly reflects the cost of providing such plans. He noted unlimited-plan customers "aren't the majority" for the carrier.

Customers under the old unlimited pricing won't have to pay more unless they switch plans. Subscribers to T-Mobile's tiered plans will automatically receive the increased data allotments.

Regulators have praised T-Mobile for making the U.S. market more competitive. But others in the industry have questioned how long the company can afford to keep up the pressure.

The carrier capped 2013 with a big increase in subscribers and projected more gains this year. But the cost of winning that business hurt its margins and widened its loss in the fourth quarter.

T-Mobile executives have been at pains to say they aren't planning a price war--particularly given the heavy cost of rolling out faster network technology called LTE--and in fact are collecting more money from subscribers.

"I don't think it's an environment that would result in an all-out price war," T-Mobile finance chief Braxton Carter said in an interview last month. "There wouldn't be adequate capital to support the build out."

T-Mobile said data demand has been soaring under its existing plans. Overall, the company said its subscribers use nearly 50% more data now than they did a year ago. Monthly usage on unlimited 4G LTE plans has nearly doubled and tops 5 gigabytes.

Write to Thomas Gryta at thomas.gryta@wsj.com

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