NEW YORK (Bloomberg) — Bermuda-based Assured Guaranty Ltd., the bond insurer with three stable AAA credit ratings, posted a $169.2 million loss as the mortgage-linked securities it guarantees fell in value.

The first-quarter net loss was $2.11 a share, compared with net income of $39 million, or 57 cents a share, a year earlier, the company said yesterday in a statement. Profit excluding the reduction in the value of guarantees fell to $6.2 million from $46.1 million a year earlier. New business production more than doubled to $276.6 million, the company said.

Assured Guaranty and Financial Security Assurance Holdings Ltd., owned by Dexia SA, are the only top-ranked bond insurers that have managed to hold stable outlooks on their AAA credit ratings amid sweeping downgrades of securities backed by home loans. That's allowed Assured Guaranty to take market share from MBIA Inc. and Ambac Financial Group Inc., the bond insurers with the largest amount of guarantees outstanding.

Investor Wilbur Ross disclosed yesterday that his investment firm WL Ross & Co. holds a 13.4 percent stake in the company.

On April 25, billionaire Warren Buffett's new bond insurer, Berkshire Hathaway Assurance Corp., received an Aaa rating from Moody's Investors Service, expanding the ranks of top-rated insurers.

Assured Guaranty fell 39 cents to $25.39 yesterday in New York Stock Exchange composite trading and is down about 17 percent in the past year.

Dominic Frederico, president and chief executive officer of Assured Guaranty Ltd. said: "Although this quarter we achieved the highest new business production for our direct segment in Assured's history, which shows a strong demand for our financial guaranties, we further downgraded some US RMBS (residential mortgage-backed securities) exposures, in particular those related to home equity lines of credit, which generated loss reserves that offset our operating income in the quarter.

"These further downgrades reflect our uncertainty related to the performance variables embedded in these exposures such as draw rates, prepayment speeds and future default rates."