A judge has ordered a temporary halt to the takeover of Warner Bros Discovery (WBD) by Paramount Skydance, as several US states move to block the deal.
The ruling, which applies for 14 days, was handed down by a US district judge ahead of a hearing in August.
That is set to determine whether the $110bn (£81.4bn) deal should be paused pending the outcome of the lawsuit brought by the 12 states involved, led by California.
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They argue the merger - mainly aimed at creating a bigger rival to the likes of Netflix - would substantially weaken competition in the US market, driving up prices for consumers and costing jobs.
The opposition is particularly fierce in Hollywood. Movie industry interest groups claim the deal would threaten the very future of cinema as it would bring together two of its five major studios.
The UK government has also outlined concerns.
The companies control UK-based TV channels, including Channel 5, TNT Sports, Cartoon Network, Nickelodeon, and CNN International, as well as the Paramount+ and HBO Max streaming services.
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Neither Paramount or WBD were yet to comment on the US ruling, which was handed down at the Oakland federal court in California.
The state's attorney general, Rob Bonta, said in a statement: "This is a critical first win in our case to ensure this megamerger never sees the light of day.
"History tells the tale of what happens when a few people have great power over markets that are central to Americans' lives: fewer opportunities for more people, worse products and services for all people."
The lawsuit could take many months to reach a conclusion - something that will worry Paramount.
Under the terms of its deal, it is obliged to pay Warner Bros shareholders a 25 cents-per-share fee for each calendar day the merger is delayed beyond 30 September.
It amounts to a bill of around $7m a day.
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