Sprint, the No. 3 U.S. mobile carrier, said in a statement on Monday that the transaction would harm consumers and competition.
Sprint will fight this attempt by AT&T to undo the progress of the past 25 years and create a new Ma Bell duopoly, said Vonya McCann, Sprint's senior vice president of government affairs.
Sprint already faces tough competition from its two bigger rivals, AT&T and Verizon Wireless, a venture of Verizon Communications
The U.S. Federal Communications Commission and the Justice Department are expected to take at least a year to review the proposed merger.
(Reporting by Jasmin Melvin; Editing by Tim Dobbyn)