WASHINGTON — The Justice Department filed suit Wednesday to block AT&T’s $39 billion deal to buy T-Mobile USA on grounds that it would raise prices for consumers.

The government contends that the acquisition of the No. 4 wireless carrier in the country by No. 2 AT&T would reduce competition.

At a news conference, Deputy Attorney General James Cole said the combination would result in “tens of millions of consumers all across the United States facing higher prices, fewer choices and lower quality products for mobile wireless services.”

The lawsuit seeks to ensure that everyone can continue to receive the benefits of competition, said Cole.

AT&T said it would fight and ask for an expedited court hearing “so the enormous benefits of this merger can be fully reviewed.” The company said the government “has the burden of proving alleged anti-competitive effects, and we intend to vigorously contest this matter in court.”

Four nationwide providers — Verizon, AT&T, T-Mobile and Sprint — account for more than 90 percent of mobile wireless connections.

T-Mobile has been an important source of competition, including through innovation and quality enhancements such as the rollout of the first nationwide high-speed data network, according to Sharis Pozen, acting chief of Justice’s antitrust division.

Mobile wireless telecom services play an increasing role in day-to-day communications, with more than 300 million smartphones, data cards, tablets and other mobile wireless devices in use.

Deutsche Telekom, the owner of T-Mobile, had no immediate comment.

AT&T and T-Mobile compete nationwide, in 97 of the largest 100 cellular marketing areas, according to the suit filed in U.S. District Court in Washington.

It says AT&T’s acquisition of T-Mobile would eliminate a company that has been a competitive factor through low pricing and innovation.