UnitedHealth participates in the Affordable Care Act in 34 states, where it provides individual health plans through online exchanges for government-backed medical coverage.

The health carrier said a week ago that it would exit Arkansas, Georgia and Michigan — after warning last November it would stop selling policies in states where it was losing money.

State-based exchanges are a foundation element to ObamaCare.

UnitedHealth put last year’s loss from ACA exchanges at $475 million. And on Tuesday, while reporting a better-than-expected first quarter, President Dave Wichmann said exchange losses in the year to date have already amounted to $125 million.

The company added that it ended the quarter with 795,000 public-exchange enrollees — a number it hopes to cut to 650,000 by December.

UnitedHealth, like several other insurers, has complained about states with too many unhealthy customers who, in turn, file unexpectedly high medical claims.

CEO Stephen Hemsley returned to that theme on Tuesday, calling small markets populated with high-risk enrollees a bad mix. “Our own experience and performance have been unfavorable in these markets,” he said.

Wall Street applauded UnitedHealth’s reduced commitment to ObamaCare, sending the stock up 2.1 percent, to $130.50.

Merrill Lynch analyst Kevin Fischbeck also praised the move, saying the insurer “has taken this volatile part of its business and apparently mitigated the future earnings risk.”

ObamaCare represents a small fraction of UnitedHealth’s business, which mostly consists of providing medical coverage to millions of people through their employers. But the company’s retreat could be a blow to President Obama’s quest to reform the health-care industry.

A study by the Kaiser Family Foundation found that if UnitedHealth exited from the 1,855 counties where it competes, 29 percent would see the number of exchange insurers drop from two to one. Another 29 percent would see the number drop from three to two.

More competition among insurers generally means lower premiums. But this is mitigated by UnitedHealth’s reputation for not being a low-cost provider.

“Even when it did price relatively low, it was often not significantly lower than its nearest competitors,” the foundation said. “As a result, the effect of a United withdrawal nationally would be modest.”

More worrisome is industry consolidation.

Third-ranked Aetna reported in January that it expects to close on its $37 billion acquisition of Humana after receiving Justice Dept. approval this year. And second-ranked Anthem also awaits regulatory approval for its $54 billion purchase of Cigna.

Should both mergers go through, the industry would go from five big players to three.

And that would make ObamaCare all the more vulnerable if another carrier also decides to leave.