In a mega-deal that alarms some health policy experts and consumer advocates worried about increasing industry consolidation and the prospect of higher premiums, health insurance giant Anthem has agreed to acquire rival Cigna for $54 billion.

The agreement announced Friday would create the health insurance industry's biggest company by enrollment and capped weeks of frenzied deal-making in the health care sector.

Aetna this month reached a $37 billion deal to buy Humana. And Southern California-based Health Net has agreed to be acquired by Centene for $6.8 billion.

The merger of Anthem and Cigna would create a company with about $115 billion in annual revenue and 53 million members. That would make it the largest U.S. health insurer in terms of membership, ahead of industry leader United Health Group, which has 46 million members.

The Aetna-Humana combination would have about 33 million members, rounding out a dominant big three in the health insurance business.

Antitrust authorities are expected to thoroughly review this new industry landscape.

The proposed merger must not only be scrutinized by federal government regulators, but also by insurance regulators in states where the health plans are sold, to ensure that consumers and health care providers are protected from abuse by mega-insurers.

"The motivation for these mergers is how they'll help shareholders. What they might mean for consumers is entirely incidental," Larry Levitt, a vice president at the Menlo Park-based Kaiser Family Foundation, said Friday.

"Less competition rarely benefits consumers in the end, though the insurance market has some quirks that make that judgment complicated," Levitt added.

With hospitals consolidating and often becoming more efficient, he said, the big new insurers might be able to negotiate high reimbursements and thus lower premiums.

But, he cautioned, "whether insurers will pass along the benefits of lower prices to consumers and employers is an open question."

Levitt and others also noted that the Affordable Care Act, also known as Obamacare, includes protections that limit the ability of insurers to gain windfall profits from less competition.

The law requires insurers that fail to spend a certain share of their premiums revenue on health care expenses for patients to give rebates to consumers and employers. The threshold is 80 percent for individuals and small businesses and 85 percent for large businesses.

But Jerry Flanagan, a lead staff attorney at the Santa Monica-based advocacy group Consumer Watchdog, said consumers shouldn't hold their breath waiting for any rebate checks in the mail.

"History shows us that when health insurance companies merge, premium costs go up," said Flanagan, adding that consumers also should expect higher deductibles and co-pays, something he called "hidden premiums," if the deal is ultimately approved.

In California, health plans are regulated by the state Department of Insurance and Department of Managed Health Care. Each has the ability to impose requirements on a merger before it is accepted here.

In 2005, for example, California regulators approved United Health Group's $8.1 billion acquisition of PacifiCare Health Systems after executives assured regulators that the merger costs would not be borne by any of PacifiCare's customers.

Regulators in 2004 approved Anthem's then-$17.5 billion acquisition of WellPoint Health Networks, as long as the new company committed $265 million in contributions, grants and investments for low-income communities in the state.

California Insurance Commissioner Dave Jones on Friday said his office will "carefully review" the proposed Anthem-Cigna merger and any others.

Anthem is California's largest for-profit health insurer and also sells Blue Cross policies in 13 other states.

"Anthem already dominates the employer-based insurance markets in 10 of the 14 states where it owns Blue Cross Blue Shield plans, and the addition of Cigna leaves employers in those states with even less choice than they have now," said Paula Wade, an analyst at Decision Resources Group in Burlington, Massachusetts.

Anthem, however, expressed confidence it can obtain the necessary regulatory approvals and expects the merger to close in the second half of 2016.

"We believe this is the best transaction for both companies," Joseph Swedish, Anthem's chief executive, said in a conference call Friday with analysts. "We will create a company that will transform health care and benefits for consumers."

The talks between Anthem and Cigna spilled into public view last month as Anthem expressed frustration with Cigna's demands. In response, Cigna criticized Anthem's bid and negotiating tactics.

Besides regulators, another hurdle for Anthem is potential opposition from its fellow Blue Cross and Blue Shield plans. Cigna officials raised that issue themselves last month, questioning how Anthem would do business in states where another insurer is the local Blues plan.

During a conference call Friday, Swedish expressed confidence that those details can be resolved and won't impede completion of the merger.

"I am optimistic we will develop a model that works for all of Blue," he said. "It's premature to go into any more details."

Staff writer Tracy Seipel and the Los Angeles Times contributed to this report.