NEW YORK (Bloomberg) — The Aaa financial strength ratings of bond insurers MBIA Inc. and Ambac Financial Corp. face a new threat from Moody's Investors Service as losses from the mortgage-market slump deepen.

Moody's yesterday put the ratings under review for a downgrade for the second time this year, dragging Ambac stock to a record low and shares of MBIA to the lowest since June 1998. MBIA chief executive officer Jay Brown rebuked Moody's for an "unnecessary" review, and Ambac CEO Michael Callen said the timing was "unfortunate" because the company's problems are temporary.

Moody's analyst Jack Dorer said a downgrade is "the most likely outcome" of the reviews, citing diminished "new business prospects and financial flexibility" as well as the likelihood the companies will report bigger insurance losses. The two biggest bond insurers sold a combined $4.1 billion in shares, bonds and convertible debt to bolster capital and save their ratings. With shares of both companies down more than 90 percent in the past year, raising more money may not be possible, analysts said.

"These companies are getting hit from all sides," said Robert Haines, an analyst with CreditSights Inc., an independent bond research firm in New York. They "aren't writing new business, they're going to have more losses and they can't access the market to replenish capital. How can they be triple-A rated?"

Ambac had a $1.66 billion net loss in the first quarter after $3.1 billion in charges for sub-prime-mortgage securities that it insured. MBIA's loss was $2.4 billion as the value of derivatives it sells to guarantee debt tumbled $3.58 billion. The financial strength rating of MBIA's main insurance unit, MBIA Insurance Corp., likely will fall to the Aa range, although a drop to the A category is possible, Moody's said in a statement. Ambac Assurance Corp.'s will probably be lowered to Aa, the company said.

Moody's and Standard & Poor's put the ratings of New York-based Ambac and Armonk, New York-based MBIA under review for the first time in January. The prospect of downgrades then roiled world capital markets on concern that the companies' guarantees for more than $1 trillion of debt may be worthless. While Fitch Ratings cut Ambac to AA in January and MBIA to AA in April, Moody's affirmed the Aaa ratings of MBIA in February and Ambac in March.

MBIA said yesterday it had the impression in February that it had six to 12 months before Moody's ratings would come under review.