The US Department of Justice has filed a lawsuit to block telecoms giant AT&T’s $85.4bn acquisition of Time Warner, the owner of CNN and HBO.
The decision is a surprise since US anti-trust authorities rarely object to deals involving companies that do not directly compete.
AT&T has said it will fight the decision.
US President Donald Trump objected to the deal during his campaign last year.
AT&T’s general counsel David McAfee said: “Today’s DOJ lawsuit is a radical and inexplicable departure from decades of antitrust precedent.
“Vertical mergers like this one are routinely approved because they benefit consumers without removing any competitor from the market. We see no legitimate reason for our merger to be treated differently.”
Mr McAfee added: “We are confident that the court will reject the government’s claims and permit this merger under longstanding legal precedent.”
US officials gave a hint of their intentions last week.
In a speech, Makan Delrahim, assistant attorney general of the DoJ’s anti-trust division, said he did not favour the remedies the department has used in the past to curb anti-competitive behaviour.
He said agreements that allow deals to go ahead, but require ongoing monitoring by the Department of Justice are overly intrusive and hard to enforce.
Mr Delrahim was appointed by President Donald Trump and sworn into office last month.
Previously, US media reported that the Department of Justice was pushing AT&T to sell some of its assets as a condition for approval. The options included CNN or its satellite dish network.
AT&T chief executive Randall Stephenson said last week he would not sell CNN.