US mobile network provider Sprint Nextel has asked US regulators to block AT&T's takeover of T-Mobile USA, saying the deal would harm competition.
Sprint has been the most vocal opponent of the $39bn (£24bn) deal that would create the largest US wireless network.
It argued the deal had "no public interest benefit", in a filing at the Federal Communications Commission.
Forcing AT&T to divest assets would not be enough to prevent "serious anti-competitive harms", it added.
"This proposed takeover puts our mobile broadband future at a crossroads," said Sprint's Vonya McCann in a statement.
"We can choose the open, competitive road best travelled, and protect American consumers, innovation and our economy, or we can choose the dead end that merely protects only AT&T and leads the rest of us back down the dirt road to Ma Bell."
Sprint, the third biggest mobile operator in the US, also said the deal would lead to higher prices for consumers.
AT&T agreed to buy T-Mobile from Deutsche Telekom in March, but the deal still needs approval from regulators.
If approved, it would give AT&T about 43% of the US mobile market, taking it ahead of industry leader Verizon Wireless.