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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.
The combination of AT&T and T-Mobile USA, the No. 4 U.S. operator, would leapfrog Verizon Wireless as market leader.