TIMELINE: Bond insurers' ratings

NEW YORK (Reuters) - Troubles in the $2.4 trillion bond insurance industry could come to a head soon as major rating agencies are expected to make decisions on the top credit ratings of the largest insurers.

Downgrades would be a serious blow for bond insurers, which need top "AAA" ratings to win business, and for banks and brokers that entered credit derivatives deals with the insurers. Financial institutions could face an additional $70 billion of write-downs if the insurers' ratings are cut, Oppenheimer & Co. has estimated.

Though some of the bond insurers have already been downgraded, key decisions are still awaited from Moody's Investors Service and Standard & Poor's on the two largest insurers: MBIA Inc.'s (MBI.N) MBIA Insurance Corp. and Ambac Financial Group Inc's (ABK.N) Ambac Assurance Corp.

If those bond insurers lose their top ratings from Moody's and S&P, so could much of the $1.2 trillion of municipal and consumer debt they insure, sparking investor losses and potentially lifting borrowing costs and dragging on economic growth, analysts have said.

Moody's and S&P recently put both Ambac and MBIA on review for downgrade, sending shivers through Wall Street, buffeting stocks, and sending insurance regulators scrambling to orchestrate a rescue.

Following is a chronology of recent rating actions on bond insurers and ratings still under review for possible downgrades: MBIA Inc.'s MBIA Insurance Corp

February 5 - Fitch puts AAA rating on review for downgrade

January 31 - S&P puts AAA rating on review for downgrade

January 17 - Moody's puts Aaa rating on review for downgrade

January 16 - Fitch affirms AAA rating with stable outlook

December 19 - S&P changes outlook on AAA rating to negative from stable

December 14 - Moody's affirms Aaa rating, changes outlook to

negative from stable Ambac Financial Group's Ambac Assurance Corp

January 18 - Fitch cuts AAA rating to AA

January 18 - S&P puts AAA rating on review for downgrade

January 16 - Moody's puts Aaa rating on review for downgrade