NEW YORK -- U.S. antitrust officials will file lawsuits to stop the two large health insurance deals they have been scrutinizing for a year, Anthem Inc.'s acquisition of Cigna Corp. and Aetna Inc.'s takeover of Humana Inc., a person familiar with the matter told Reuters.

Antitrust regulators have been concerned that consolidation of the nation's largest insurers would raise prices for Americans. Anthem and Aetna have said the deals will help consumers by giving the companies the scale to create more cost savings for customers.

The department's decision could be announced by the end of next week, the source familiar with the matter added.

Since the deals were announced a year ago, investors have doubted that the $48 billion Anthem-Cigna combination would go through. Both companies manage health benefits for the nation's employers, who sponsor insurance for more than 150 million people.

Anthem officials said in May that the Justice Department was particularly concerned about how the purchase would affect large companies with employees nationwide. Analysts have said it would be difficult to sell assets to a competitor to answer those concerns.

Until recent weeks, investors were betting that the $34 billion Aetna-Humana combination was less troublesome and could be approved if the companies sold some Medicare assets. But as the Justice Department's "significant concerns" about both deals became public earlier this month, expectations rose that the antitrust regulators would sue to block that transaction as well .

Aetna's purchase of Humana would combine two of the largest providers of Medicare Advantage plans for elderly people. Sources have told Reuters that the companies had presented a divestment plan and the names of possible buyers to the Justice Department less than two weeks ago.

Anthem, Cigna and Aetna declined to comment. Humana and the Justice Department were not immediately available for comment.

Aetna shares fell 4 percent to $113.40 and Humana fell 5 percent to $151.49. Anthem fell 3 percent to $130.93 and Cigna fell 2.4 percent to $129.94.

The two deals were announced after years in which the health insurers were contending with new costs and taxes associated with President Barack Obama's national healthcare reform law, called the Affordable Care Act, as the companies said they need more scale to compete.

The deals would reduce the number of large insurers to three from the current five. Minnetonka-based UnitedHealth Group Inc, now the largest insurer, would have ranked as the second largest between Anthem and Aetna.

Leerink Partners analyst Ana Gupte said in a research note that she expects the companies to fight the Department of Justice, but that Anthem's and Cigna's chances of success were slim.

Indeed, a source familiar with the situation told Reuters earlier this month that Aetna was prepared to fight the Justice Department.

If there are no deals, Gupte said she expects Humana and Cigna to use their strong cash positions to buy back shares or make small acquisitions. WellCare Health Plans, Molina Healthcare Inc and Centene Corp are possible targets, she said, though only WellCare is potentially open to a deal.

Bloomberg first reported that the Justice Department had decided to sue to block the mergers as soon as this week.