One of the two giant health insurance mergers announced in 2015 has been blocked by a federal judge.

The proposed merger between Aetna Inc. and Humana Inc. would "substantially lessen" competition for individual Medicare Advantage products in every county cited in a Department of Justice lawsuit brought to stop the merger, District Judge John Bates wrote in his Jan. 23 opinion.

"This conclusion is based on identification of the proper product market, the overwhelming market concentration figures generated by the merger, and the considerable evidence of valuable head-to-head competition between Aetna and Humana, which the merger would eliminate," Bates wrote.

The court reached that conclusion despite Aetna and Humana agreeing in August 2016 to sell much of their Medicare Advantage assets to Molina Healthcare Inc. Bates' opinion appears to provide an answer to the so-called "relevant market" question, which asks whether Medicare Advantage is its own product and has its own market, or participates in and competes in the broader Medicare market.

Bates also said the merger would substantially lessen competition on the public exchanges in three counties in Florida. Aetna's decision to withdraw from several public insurance exchanges in Florida was made in order to avoid antitrust scrutiny, the judge said.

"The Court gives that evidence little weight in predicting whether Aetna will continue to compete on the exchanges in the future," Bates said.

Aetna CEO Mark Bertolini has defended the company's decision to exit many Affordable Care Act exchanges as a response to losses created by operating in them, rather than to lessen the company's exposure to anticompetitive scrutiny. Several U.S. senators, including Elizabeth Warren, D-Mass., accused the company of withdrawing from exchanges to pressure the Justice Department into approving the combination with Humana.

Bates was also unconvinced that the efficiencies created by the merger would mitigate the anticompetitive effects that would be felt by consumers.

According to a regulatory filing made at the time of the merger's announcement, Humana may be required to pay Aetna a termination fee of $1.31 billion, while Aetna may be required to pay Humana $1.69 billion. Aetna could be on the hook for another $1.00 billion termination fee to Humana if the merger is terminated by either party due to legal constraints related to required regulatory approvals prohibiting the deal.

T.J. Crawford, an Aetna spokesman, said the company is reviewing the opinion now and giving serious consideration to an appeal.

The opinion on another DOJ lawsuit seeking to stop the merger between Anthem Inc. and Cigna Corp. is still pending.