SAN FRANCISCO (MarketWatch) -- Just when you thought it was over, trouble in the $2.3 trillion bond-insurance business could trigger another wave of big write-downs from banks and brokerage firms, experts said Friday.

Leading bond insurers Ambac Financial ABK, and MBIA Inc. MBI, -4.41% look increasingly likely to lose their AAA ratings. While almost unthinkable just six months ago, such concerns are also causing turmoil in the $2.5 trillion municipal-bond market.

Bond insurers agree to pay principal and interest when due in a timely manner in the event of a default -- a $2.3 trillion business that offers a credit-rating boost to municipalities and other issuers that don't have AAA ratings. Without those top ratings, their business models may be imperiled.

A more worrying consideration is that when a bond insurer is downgraded, all the securities it has guaranteed are, in theory, downgraded as well.

“ 'The destruction of the bond insurers would likely bring write-downs at major banks and financial institutions that would put current write-downs to shame.' ” — Tamara Kravec, Banc of America Securities

If Ambac and MBIA lose their top ratings, billions of dollars of muni bonds will be downgraded, and the guarantees that have been sold on mortgage-related securities such as collateralized debt obligations, or CDOs, will lose value.

Bond insurers guarantee roughly $1.4 trillion worth of muni bonds and more than $600 billion of structured finance securities, such as mortgage-backed securities and CDOs, according to Standard & Poor's. Ambac alone has guaranteed about $67 billion of CDOs.

"The destruction of the bond insurers would likely bring write-downs at major banks and financial institutions that would put current write-downs to shame," Tamara Kravec, an analyst at Banc of America Securities, wrote in a note Friday.

Kravec cut her rating on Ambac and MBIA on Friday because she thinks that ratings downgrades are "highly probable" now.

Indeed, Fitch Ratings cut Ambac's AAA rating to AA on Friday, becoming the first major agency to take that step. Fitch downgraded 137,390 muni bond issues and 114 other securities guaranteed by Ambac soon after.

Merrill Lynch & Co. MER, +27.69% took a $3.1 billion write-down on Thursday related to the firm's CDO hedges. Merrill had bought CDO guarantees from bond insurers including ACA Capital, a smaller player that's now struggling to survive. Most of the write-downs were related to ACA. See full story.

CIBC CM, -1.44% and French banking giant Credit Agricole unveiled similar write-downs in December, related to guarantees they bought from ACA. See story on ACA troubles.

'Whopping'

But ACA is much smaller than Ambac and MBIA. If the two larger bond insurers are downgraded, banks and brokers that have bought guarantees from them may have to write-down their exposures further.

Merrill has net CDO exposure of $4.8 billion. But that includes a lot of hedging, mainly through guarantees bought from bond insurers. Excluding those hedges, the brokerage firm still has a "whopping" $30.4 billion of CDOs on its balance sheet, Brad Hintz, an analyst at Bernstein Research, noted on Friday.

"We remain very uncomfortable with Merrill's CDO balance sheet exposure," the analyst wrote in a note to investors. "If the counterparties are downgraded, and they cannot post additional collateral, we would expect that Merrill Lynch would have to take a valuation reserve against that specific exposure."

Citigroup C, -2.12% set aside $900 million during the fourth quarter to cover heightened credit risks related to counterparties it uses to hedge CDO risks.

Muni bond turmoil

The impact on the muni-bond market may be just as big, experts said Friday.

There are $2.5 trillion to $3 trillion of muni bonds. Roughly half of those are insured by bond, or "monoline," insurers like Ambac and MBIA.

So more than $1 trillion of muni bonds are now in danger of being downgraded. That could trigger losses for muni-bond investors.

"Assuming the "monoline" insurers lose their triple-A ratings, underlying insured muni bonds could be susceptible to downgrades and downward repricing, leading to losses for muni-bond mutual funds," Michael Kim, an analyst at Sandler O'Neill, told investors in a note Friday.

Shares of big muni-bond fund managers, including Franklin Resources BEN, -0.79% and Eaton Vance EV, -0.83% have already been hit by such concerns, Kim said.

Franklin stock has slumped 22% so far this year; Eaton Vance is off 27%.

Information vacuum

Most muni bonds insured by Ambac and MBIA are now trading as if there isn't any insurance, Richard Larkin, a municipal-trading desk analyst at JB Hanauer & Co., commented Friday.

"The market has lost all faith in bond insurance and the ratings agencies," he said. "Prices are being discounted because people wonder whether there is any value to the insurance."

That's a big problem, because there are no official ratings for many of the underlying issuers of muni bonds, such as cities, school districts and utilities, he added.

When municipalities sold debt, they asked agencies like S&P and Moody's to evaluate the securities with bond insurance attached.

“ 'The market has lost all faith in bond insurance and the ratings agencies. Prices are being discounted because people wonder whether there is any value to the insurance.' ” — Richard Larkin, JB Hanauer & Co.

If the insurance on this debt becomes less valuable, muni bond investors have few ways of checking the new creditworthiness of the issuer, Larkin said.

That's creating an "information vacuum" because the rating agencies aren't going to re-evaluate muni bond issuers unless the municipalities request and pay for new analysis, he said.

"The lack of public underlying ratings on insured debt is a big problem, and if more bond insurers are downgraded, the rating agencies are not likely to fill the vacuum and publish underlying ratings unless they are paid additional fees to do so," Larkin explained.

"Trades are being made based on people's best guesses of the creditworthiness of issuers," he added. "And if these downgrades happen, that will be the environment going forward. Not a good one."