The Obama administration on Thursday filed lawsuits against two multibillion-dollar health insurer mergers, warning the takeovers would drastically threaten competition nationwide.

In a landmark antitrust move, the Justice Department is suing to block both Anthem’s $54 billion takeover of Cigna and Aetna’s $37 billion bid to buy Humana.

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The massive mergers would turn the nation's five biggest insurers into just three, a fact Attorney General Loretta Lynch frequently mentioned as she announced the move Thursday.

“If allowed to proceed, these mergers would fundamentally reshape the health insurance industry,” she said at a briefing. “They would leave much of the multitrillion-dollar industry in the hands of just three mammoth insurers.”

“These mergers may indeed increase the profits of Aetna and Anthem, but it would do so at the expense of consumers,” she added.

The cases remain separate, but would be decided by the same judge, she said.

In sharply worded statements on Thursday, officials from both Aetna and Anthem said they would fight in court to move ahead with their mergers.

Aetna said it would “vigorously” defend its takeover bid of Humana, arguing that a combined company “offers tremendous value to consumers.”

Anthem, which has sought to acquire Cigna, said it is “fully committed to challenging the [Department of Justice] DOJ’s decision in court.” It blasted the department for its “flawed analysis and misunderstanding of the dynamic, competitive and highly regulated healthcare landscape."

The Hartford, Conn.-based company also made clear that it would be “receptive to any efforts to reach a settlement.” Experts say that could involve selling off small pieces of the company that raised concerns about competition.

The lawsuits against the proposed mergers are the culmination of more than a year of investigations by the Department of Justice.

‘We have no doubt these mergers would reduce competition over what it is today,” William Baer, the principal deputy associate attorney general, said at the press conference.

Updated 12:26 p.m.