UnitedHealth Group Inc. (UNH) said Thursday it may have to restate financial results for the past three years to lower earnings by as much as $286 million because of the way it accounted for stock options granted to executives.

The nation's second-largest health insurer also acknowledged it is the subject of an "informal inquiry" by the Securities and Exchange Commission, and it could lose tax deductions for options grants it previously thought were deductible.

Chairman and CEO William McGuire had more than $1.6 billion in unexercised options at the end of last year, and questions have been raised about the timing of some of them.

The disclosure on Thursday marks the first time UnitedHealth has admitted there may be a problem in the way it handled stock options. It had previously described its practices as "appropriate" and had said a committee of its board of directors is looking into the issue.

Of a possible total of $286 million from the three years, $150 million is from 2005. That would represent 4.5 percent of its net earnings that year, the company said. The possible restatements for 2003 and 2004 would add up to roughly 3 percent of each year's earnings, it said.

UnitedHealth said it believes the potential lost tax deductions from earlier years would not have a large impact on its results, but said it "will not be able to finalize its assessment of this matter until" the investigation by its board is finished.