A federal judge on Thursday rejected a U.S. government effort to disqualify a lawyer representing 15 states and the District of Columbia in their effort to block T-Mobile US' deal to buy Sprint Corp.

U.S. Magistrate Judge Robert Lehrburger said the Department of Justice waited too long to seek the disqualification of Glenn Pomerantz, who had represented the department in 2011 when it stopped AT&T's purchase of T-Mobile. He also said Pomerantz's firm, Munger, Tolles & Olson LLP, could stay on the case.

The decision was made in a hearing before a Dec. 9 trial to determine whether T-Mobile, the third-largest U.S. wireless carrier, may go forward with its planned $26.5 billion merger with smaller rival Sprint Corp, the fourth-largest wireless carrier.

State attorneys general from 15 states and the District of Columbia argue that the deal will lead to higher prices for consumers, particularly poorer ones.

Lehrburger said the Justice Department knew in mid-April about Pomerantz's role and should not have waited until a few weeks before the scheduled trial - and after the originally scheduled October trial date - to speak up.

"The division's application is untimely," the Manhattan judge said, referring to the Justice Department's Antitrust Division. "The division could have and should have moved much earlier."