Most of those eligible for health insurance subsidies under the Affordable Care Act are failing to claim them, according to a new study.

Researchers with the Robert Wood Johnson Foundation and the Urban Institute estimated that more than 24 million people were eligible for ObamaCare tax credits last year. By March, only 41 percent of them had selected a plan on a government insurance exchange.

By June, the study said, just 35 percent of them had actually enrolled.

“We need perhaps twice as many people signing up in order to be assured of getting a good spread of risk, and therefore, a sustainable cost for people,” said Robert Laszewski, the president of Health Policy and Strategy Associates.

Researchers said many of those eligible were unaware of the benefits.

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“In our surveys, we found recently that of the uninsured people who are probably eligible for tax credits but not enrolled, about half of them said that they had not heard of the tax credits, and had not visited healthcare.gov or their state site,” said Matthew Buettgens, a senior research analyst in the Health Policy Center at the Urban Institute.

“There is a lot more to be done, in getting people who are eligible for these tax credits to even know that it’s a choice for them.”

He added that many of those eligible for tax credits may have purchased insurance without applying for them, and are unnecessarily bearing costs the government is willing to pay.

The study also found those receiving less financial help from the government were less inclined to purchase insurance on the exchanges because they found coverage too expensive. 

“The limits of this willingness to pay could make increases in future enrollment difficult to achieve without policy changes to improve affordability,” noted the study.

Premium prices are also increasing.The "silver plan," the second-cheapest available on the federal exchange, will cost on average 7.5 percent more next year, according to administration figures.

“Consumers are faced with higher premiums, higher deductibles, and narrow networks, rate increases we're looking at only tell part of the story,” said Laszewski.

For single adults earning more than $47,000 annually, or about $100,000 for a family of four, there are no tax credits.

With government subsidies, most consumers can find plans for less than $100 a month, the administration said. ObamaCare supporters said the law offers better coverage and its primary goal is to reduce the number of uninsured. More than 17.5 million have gained health insurance because of ObamaCare, according to the Department of Health and Human Services. 

Americans started signing up for 2016 ObamaCare coverage earlier this month.They have until the end of January to select a plan and apply for tax credits. 

Rich Edson is a Washington correspondent for Fox News Channel. Prior to that, he served as Fox Business Network's Washington correspondent.