The deal announced more than a year ago would merge the vast content of Time Warner units like premium cable channel HBO and news channel CNN with the massive internet and pay TV delivery networks of AT&T.
"This merger would greatly harm American consumers. It would mean higher monthly television bills and fewer of the new, emerging innovative options that consumers are beginning to enjoy," said Makan Delrahim, head of the Justice Department's antitrust division.
Delrahim said AT&T with its DirecTV satellite operations and Time Warner's content "would have the incentive and ability to charge more for Time Warner's popular networks and take other actions to discourage future competitors from entering the marketplace altogether."
Critics of the deal have said it would give too much power over the media industry to a single firm and enable AT&T to withhold key content from rivals or raise prices.
AT&T said it plans to challenge the government's lawsuit, arguing that it was seeking a "vertical" merger without competitive overlap which should be approved based on legal precedent.
Randall Stephenson, AT&T's chairman and chief executive, said the antitrust enforcers are ignoring "decades of clear legal precedent" and failed to take into account the "radical change" in the sector in which internet platforms like Netflix are transforming how media is consumed.
The deal has also stirred up political concerns: Reports earlier this month said the government was prepared to approve the deal if AT&T would divest CNN, which has been a frequent target of President Donald Trump, who has attacked the network as "fake news."
During the election campaign, Trump vowed to block the merger that would have some 142 million subscribers and a vast catalogue of television, film and sports content.
Stephenson, at a news conference, reaffirmed his opposition to divesting CNN to win approval.
"There's been a lot of reporting and speculation whether this is all about CNN, and frankly I don't know," he said.
"But nobody should be surprised that the question keeps coming up because we have witnessed such an abrupt change in the application of antitrust law.
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