The Justice Department's appeal is lodged with the U.S. Court of Appeals for the District of Columbia Circuit, a step up from the federal district court where the six-week trial unfolded in the spring. President Donald Trump, as a candidate, said, "AT&T is buying Time Warner, and thus CNN, a deal we will not approve in my administration".
The company did not immediately comment on the DoJ move.
The Department of Justice sued to block the deal in 2017, launching the biggest anti-trust suit in decades.
The US Justice Department may be the gang that can't shoot straight, but they're not out of ammo just yet in the case of AT&T and Time Warner.
Leon ruled the government had failed to meet its burden of proof that the tie-up between the largest United States pay-TV operator and the media-entertainment giant would harm competition. When that wrapped up in the beginning of May, the DOJ proposed solutions to ensure the merger wouldn't fall afoul of anti-competitive law: Either get rid of DirecTV or Turner (which contains TNT, TBS and CNN).
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Donald Trump said he opposed the deal prior to his election as U.S. president, while consumer groups have also voiced concerns over a concentration of power. "The DOJ appeal will add more noise to the story in the near term until it is heard in the courts although AT&T is moving ahead full steam in its merger plans", he said.
The appeal could be seen as a warning sign for companies that have pursued their own mergers in the wake of Leon's decision, Kimmelman added. AT&T said it was surprised by the decision and would defend the merger in court again.
The government's notice of appeal filed in US District Court in Washington did not disclose on what grounds it intends to challenge the approval.
John Bergmayer, senior counsel at advocacy group Public Knowledge, said that since the deal was approved, AT&T had raised prices for some video and wireless services.
AT&T shares slipped 1% in after-hours trade in NY.