NEW YORK (Bloomberg) ? MBIA Inc., the world?s largest bond insurer, said it may have to revise its accounting for bonds held in its investment portfolio under changes being considered by the US Securities and Exchange Commission.

MBIA, which insures bonds against default, said SEC staff told the company the changes would affect securities that carry its own guarantee. The SEC and accounting regulators are contemplating the changes for MBIA and competitors, the Armonk, New York-based company said on Friday in its annual SEC filing.

About $4.4 billion, or 14 percent, of MBIA?s holdings are MBIA-insured bonds, the company said. Because of the insurance, those bonds are rated AAA by credit-rating companies, the highest grade, and have a higher market value than lower-rated debt. Without the insurance, they would have an average rating of A, lower than the average rating of AA for the entire portfolio, MBIA said in the filing.

?This has long been discussed as a potential weakness, but this is the first I?ve heard of an effort to change the accounting,? said Mark Lane, an analyst at William Blair & Co., an investment firm in Chicago.

The issue is important for bond insurers, Lane said, because the insurers? own credit rating rests in part on the quality of securities held in its portfolio. The Financial Accounting Standards Board also is considering a change to its rules for such financial guarantees.

MBIA said it would continue to account for the MBIA-insured bonds as ?available for sale? at fair value until it receives further instructions from the SEC.

?The company cannot predict how the SEC staff will resolve this issue and the resulting impact on the company?s consolidated financial statements,? MBIA said in its filing. ?There can be no certainty, however, that the SEC will not require the company to modify its methodology, either on a going-forward basis or for prior periods.?

A spokesman for the SEC, John Nester, declined to comment in an e-mail response to an inquiry.

Ambac Financial Group Inc., the third-largest municipal bond insurer, also invests in bonds that carry the company?s own insurance pledge. Ambac on September 30 owned $840.9 million of such bonds, making up about 5 percent of its investment portfolio, the company said in a quarterly SEC filing. Ambac estimates those bonds have an average underlying credit rating of BBB+, according to the filing.

MBIA was the largest insurer of municipal bonds in 2004, backing about $53 billion of outstanding debt that year, according to the most recent data from Thomson Financial.