Leap said on Tuesday that the elimination of T-Mobile USA, a Deutsche Telekom AG unit, as a competitor would accelerate a trend towards "alarming concentration" in the wireless industry around the two biggest companies.
Another rival, MetroPCS Communications Inc, had suggested that if regulators let the deal go ahead, AT&T should be forced to divest some assets.
Sprint Nextel Corp, the No. 3 U.S. mobile provider, has been the most vociferous opponent to the merger, which would make No. 2 U.S. mobile provider AT&T the biggest U.S. operator, ahead of current leader Verizon Wireless.
Verizon Wireless is a venture of Verizon Communications Inc and Vodafone Group Plc