WASHINGTON (Reuters) - The Obama administration said on Friday it would create a special enrollment period for tax filers who were unaware they could face penalties for missing the Feb. 15 deadline to obtain health insurance through the federal marketplace.

The one-off special enrollment period will extend from March 15 to April 30. If consumers do not buy health insurance during this period they will have to pay a penalty when they file their 2015 taxes. Eligible filers must live in one of the 37 states with a federally facilitated insurance marketplace. State-based insurance exchanges can set their own policies, officials said, although they are "welcome to mirror" federal policy.

"We recognize that this is the first tax filing season where consumers may have to pay a fee or claim an exemption for not having health insurance coverage," Marilyn Tavenner, administrator of the Centers for Medicare and Medicaid Services, which oversees the insurance exchange, said in a statement.

To qualify for the special enrollment consumers must attest that when they filed their 2014 tax return they paid the fee for not having health coverage that year, and that they first became aware of the implications of not enrolling in a timely manner after they began preparing their 2014 taxes.

The IRS, or Internal Revenue Service, has estimated 2 to 4 percent of tax filers, roughly six million people, may pay a fee for not having coverage in 2014, which is $95 or 1 percent of income.

The fee increases to $325 per adult or 2 percent of income for 2015. People who enroll during the special period will still owe fees for the months they were uninsured in 2014 and 2015.

The special period is to allow people to avoid additional fees for 2015. An estimated 10 to 20 percent of taxpayers who were uninsured for all or part of 2014 will qualify for an exemption from the coverage requirement.

Officials also said IRS forms sent to about 800,000 tax filers contained a health insurance subsidy error that could cause some people to receive greater or smaller refunds than they are entitled to.

Roughly 95 percent of those who received the forms have yet to file taxes, officials said. They are being advised to wait for new forms to be sent out shortly. Those who already filed will also be notified. The erroneous forms accounted for about 20 percent of the total.

(Reporting by Toni Clarke; Editing by Lisa Lambert)