NEW YORK/WASHINGTON (Reuters) - A federal judge on Thursday rejected a U.S. government effort to disqualify a lawyer arguing for 15 states and the District of Columbia in their effort to block T-Mobile US Inc's TMUS.O planned $26.5 billion takeover of Sprint Corp S.N.

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U.S. Magistrate Judge Robert Lehrburger said the Department of Justice waited too long to intervene in the case to try to disqualify Glenn Pomerantz, who had represented the department in 2011 when it stopped AT&T’s purchase of T-Mobile, and his law firm Munger, Tolles & Olson.

Lehrburger ruled at a hearing in Manhattan federal court, less than three weeks before a scheduled Dec. 9 trial to determine whether T-Mobile, the third-largest U.S. wireless carrier, may go forward with its merger with Sprint, the fourth-largest.

Attorneys general from the 15 states and Washington, D.C. contend that the merger will boost prices and harm consumers, especially poorer ones. Pomerantz and his firm represent California, the largest of the 15 states.

Lehrburger said the Justice Department knew in mid-April about Pomerantz’s planned role and should not have waited until a few weeks before the scheduled trial - and after the original Oct. 7 trial date - to speak up.

“The division could have, and should have, moved much earlier,” Lehrburger said, referring to the Justice Department’s antitrust division.

Lehrburger, who handles many pre-trial matters in the case for U.S. District Judge Victor Marrero, said disqualifying the states’ lawyers would cause “extreme prejudice” to both sides in a “very complex” merger case.

He also said it would prejudice the public, who had an interest in seeing a resolution “sooner rather than later.”

Lawrence Reicher, a lawyer for the U.S. government, had argued that there was a “substantial relationship” between Pomerantz’s work in 2011, when he had access to the government’s files, and the current case.

“The United States’ paramount interest is preserving our confidential information,” he said.

T-Mobile’s purchase of Sprint has won conditional approval by the Justice Department and the Federal Communications Commission.

That approval requires the companies to divest Sprint’s prepaid businesses, including Boost Mobile, to satellite television company Dish Network Corp DISH.O and provide it access to 20,000 cell sites and hundreds of retail locations. That transaction is worth about $5 billion.

California Attorney General Xavier Becerra said he welcomed Lehrburger’s decision. “We will be relentless in this fight for consumer choice and real competition,” he said in a statement.