Ambac Financial Group Inc., a bond insurer that lost 93 percent of its stock market value in the past year, posted a larger loss than analysts had anticipated after $3.1 billion in charges for subprime-mortgage securities, Bloomberg reported.

The world’s second-largest bond insurer fell as much as 31 percent in early New York Stock Exchange trading after reporting a first-quarter net loss of $1.66 billion, or $11.69 a share. The company’s operating loss of $6.93 a share was more than three times the $1.82 estimated by six analysts surveyed by Bloomberg.

Ambac, MBIA Inc. and the rest of the bond insurance industry have posted record losses after expanding from their traditional business of guaranteeing municipal bonds that rarely default to securities linked to subprime mortgages that are going bad at unprecedented rates. Ambac today increased its estimate of the claims it will need to pay on home-loan debt by $2 billion.

“The housing market crisis continues to disrupt the global credit markets and our credit derivatives and direct mortgage portfolios were severely impacted once again,” Ambac interim CEO Michael Callen said in a statement today.

The company posted net income of $213.3 million, or $2.04 a share, in the first quarter of 2007, just before the subprime-mortgage market began to collapse. The housing and credit market slump that ensued since has pushed Ambac to three straight net losses after more than a decade of quarterly profits. The company posted a record loss in the fourth quarter of $3.3 billion, or $31.85 a share, largely on write-downs of $5.2 billion related to collateralized debt obligation guarantees.