Health insurance giants are eating up a bigger slice of the marketplace in most states, despite intense efforts under ObamaCare to increase competition.

The three largest insurance companies held an average of 86 percent of customers in the individual market last year, up from 83 percent in 2010, when the healthcare reform act was passed, according to a Monday report from the Government Accountability Office.

The biggest companies held at least 95 percent of all customers in nearly a dozen states, including Alabama, Iowa, Kentucky and New Jersey.

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The report found that the marketplace remained highly concentrated from 2010-2013, the latest year data was available, which shows the nation’s slow progress to dislodge long-standing insurance monopolies under the Affordable Care Act.

The findings are a blow to the Obama administration, which has touted lower costs and more options for millions across the country under the healthcare law.

Taming insurance monopolies was a major goal for Democrats shaping ObamaCare, though many health policy experts warned that it would be a marathon effort, particularly in less-populated regions where competition has historically been close to nonexistent.

While several companies saw their shares of the marketplace stay the same or decrease slightly throughout the rollout the Affordable Care Act's rollout, others gained many more customers.

In Nebraska, the largest insurer increased its share of the market from 42 percent to 61 percent over a three-year period. In Texas, the biggest company’s share ballooned from 39 percent to 53 percent.

The landscape of insurance companies could look more diverse next year, however. The report notes that some provisions of the Affordable Care Act meant to improve competition, such as rating rules, went into effect after the data were recorded.

Over the last year, ObamaCare has seen 25 percent more insurers enter the market, a move that has been hailed by HHS officials as spurring competition. About three dozen states brought in at least one new company.

The report is part of a mandated study of market competition under the healthcare law.

It was delivered to the leaders of the Senate Finance; and Health, Education, Labor, and Pensions committees and the House Energy and Commerce, Ways and Means, and Education and the Workforce committees.