"As we plan for open enrollment in year two and continue to build a sustainable long-term system, we are committed to simplifying the experience for consumers by allowing auto-enrollment," said federal Health and Human Services Secretary Sylvia Mathews Burwell.
"We are working to streamline the process for consumers wishing to remain in their current plan."
Of the remaining 5 percent who will not be eligible for the double renewal, fewer than 100,000 people opted out of automatic plan renewal when they signed up for a health plan, and a certain number of subsidy recipients were found to have incomes that now exceed 500 percent of the federal poverty level, a senior HHS official said.
But even if someone is no longer eligible for a subsidy, they will still be re-enrolled in their same plan, unless they had opted out of that automatic process, the official said.
HealthCare.gov sells insurance plans to the residents of 36 states that opted not to build their own Obamacare exchanges.
The granting of automatic subsidies, along with automatic re-enrollment, only applies to HealthCare.gov enrollees, not to the 2.7 million people who enrolled on one of the 15 exchanges run by individual states and by the District of Columbia.
Those exchanges have the option of allowing either just automatic re-enrollment in health plans, in adding the streamlined automatic renewal of subsidies for those plans, or in designing a system of their own that is subject to approval by the federal Centers for Medicare and Medicaid Services, the senior official said.
Timothy Jost, a health law professor at Washington and Lee University School of Law, said that even with automatic renewal on HealthCare.gov, "I think there will be quite a bit of movement among plans" by current ernollees.
Jost noted that a number of plans that sign-up the largest number of Obamacare enrollees on the government exchanges have announced that they plan to increase their premium prices for 2015 plans, which could lead some of those enrollees to seek less-expensive options.
"Probably a lot of people will change plans," Jost said.
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—By CNBC's Dan Mangan
Correction: An earlier version of this story indicated that the federal government was announcing the automatic renewal of subsidies for eligible people in Obamacare plans. Such renewal was included in previously announced rules, but now will be more streamlined, according to federal officials.