"Sprint urges the United States government to block this anti-competitive acquisition. This transaction will harm consumers and harm competition at a time when this country can least afford it," said Vonya McCann, senior vice president of government affairs, in a written statement on Monday.

The company said AT&T's proposed deal would roll back decades of opening up the U.S. telecommunications industry to competition following the breakup of the AT&T monopoly in the 1980s. "On behalf of our customers, our industry and our country, Sprint will fight this attempt by AT&T to undo the progress of the past 25 years and create a new Ma Bell duopoly," McCann's statement said.

AT&T but said it would probably take 12 months to conclude. The U.S. Federal Communications Commission and Department of Justice will have to approve the deal, with the FCC examining whether it is in the public interest and the DOJ ruling whether it would hurt competition. AT&T executives have said they expect to have to discuss possible divestitures of some assets.

The deal would make AT&T by far the largest mobile operator in the country, with more than 130 million subscribers, while Verizon would remain in second place with 94 million. Sprint would still be the third-biggest mobile operator in the country, with 58 million subscribers, but it would be by far the smallest major carrier if T-Mobile didn't exist.

After the buyout, AT&T would be nearly three times the size of Sprint, the company pointed out in its statement Monday. Some observers believe Sprint would be forced into focusing on low-priced devices and services in order to carve out a market niche.