UnitedHealth says it is losing big money because of the federal health care law.

UnitedHealth Group, one of the nation’s largest health insurance companies, told investors Thursday that it was significantly lowering its profit estimates and blamed an expected loss of hundreds of millions of dollars selling individual policies under the federal health care law.

In light of the losses, the Minneapolis-based company warned that it was also weighing whether it would continue to offer individual coverage through the online exchanges for 2017.

The announcement comes as the latest blow to the market created under the law to allow better access to health insurance for people who buy coverage on their own rather than through an employer. As the law enters its third year, with open enrollment for 2016 underway, many customers have f aced sticker shock as premiums have risen significantly in some parts of the country. Some of the new companies offering coverage, including the so-called coops, have stopped selling coverage in recent weeks, leaving people with fewer options.

While UnitedHealth has only been able to sell a fraction of policies to consumers through the exchanges, which some critics contend are not priced competi- tively, its discontent with the federal health care law could signal a broader industry pushback.

The insurance giant may also be using the news to prod the administration into changing the law or paying more of what the government owes insurers under one of the programs aimed at protecting them from losses in the early years. (Federal officials have said they are paying less than 13 cents of every dollar they owe, although they say they will make additional payments later.)

The company’s threatened withdrawal also puts more pressure on regulators to scrutinize the proposed mergers of its rivals Anthem with Cigna and Aetna with Humana.

Citing weak growth in the number of new people signing up for coverage and the high costs of medical care for existing customers, which it described as affecting the whole industry, the company said it was reducing its earnings estimates for 2015 and 2016.

“We cannot sustain these losses,” CEO Stephen Hemsley said Thursday. “We can’t really subsidize a marketplace that doesn’t appear at the moment to be sustaining itself.”

Hemsley refused to discuss whether the insurer had any talks with federal officials.

While UnitedHealth emphasized its other areas of business remained strong — such as offering coverage under Medicare private plans or handling claims for large employers — it said it was calculating a reduction of $425 million in profit this year because of projected losses from the online exchanges. The company said it was reducing its earnings estimate for 2015 to about $6 a share, and lowered its earnings estimates for 2016.

Obama administration officials emphasized the decision by UnitedHealth had not dampened competition in the market.

The marketplaces continue to grow, said Benjamin Wakana, a spokesman for the Department of Health and Human Services.