NEW YORK (CNNMoney.com) - IBM said Thursday that the U.S. Securities and Exchange Commission is launching a formal investigation into the way IBM disclosed last year's first-quarter earnings and into changes in the way that it would expense for stock options.

The SEC had already begun an informal probe of the matter last June, IBM said. The world's largest computer services company added that it has been cooperating with the SEC since June and will continue to do so.

Last April, IBM stunned Wall Street by reporting first quarter earnings that were significantly lower than expected. The announcement came two business days earlier than IBM had originally planned.

At the time IBM blamed soft economic conditions in the U.S. and Europe for its disappointing results. But there was also confusion about how much options expenses actually hurt profits in the quarter. IBM voluntarily agreed to start expensing options during the first quarter of last year, ahead of a rule change that will mandate the expensing of equity-based compensation for all companies.

But IBM waited until after the quarter ended to tell Wall Street that it would expense options, causing analysts to scramble to readjust their first quarter estimates. Compounding matters, when the company reported its first quarter numbers it appeared that the impact of the options expenses was lower than what IBM originally told analysts. Some analysts argued that this helped IBM make profits look better than they actually were.

Still, one analyst said Thursday that the formal SEC probe would probably not have a major impact on IBM since many investors already expected that the company would be fined.

"Moving the investigation to a formal status makes it more likely that the SEC will find fault with IBM's disclosure practice and fine them," said Richard Petersen, an analyst with Pacific Crest Securities. "But it's somewhat expected that IBM would get dinged for this so I don't think it's a big surprise, nothing earth-shattering." (Petersen doesn't own the stock and his firm has no banking ties to the company.)

Shares of IBM (Research) fell about 1 percent in after-hours trading following the news of the SEC probe. The Dow component's stock fell 60 cents, or 0.7 percent, to $83.57 in regular trading on the New York Stock Exchange.

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