MBIA, the largest bond insurer in the world, said on Thursday it is looking at ways to shore up its capital base, a day after rating agency Moody's Investors Service said the insurer was somewhat likely to require additional capital.

Bond insurers are broadly boosting their capital levels as the subprime mortgage crisis has forced them to set aside more money for losses and pay out more in claims.

The insurers have multiple ways to boost capital, including issuing securities and writing less new insurance. To the extent that the companies insure fewer bonds backed by consumer debt, borrowing costs for everything from mortgages to credit cards could rise.

Moody's said on Wednesday that a review of MBIA's residential mortgage-backed portfolio showed the company was at greater risk of falling short on capital than the agency had previously believed.

MBIA said on Thursday it has been pursuing capital contingency plans, even in the absence of any immediate rating agency requirements. Spokeswoman Liz James declined to elaborate beyond the statement.

For most bond insurers, increasing capital levels is crucial for maintaining top triple-A credit ratings, which are in turn crucial for staying in business.

Last month, bond insurer CIFG received $1.5 billion of fresh capital from Banque Populaire and Caisse d'Epargne.

Bond insurers generally have low capital levels relative to their exposure, and a few large defaults could stress their capital.

MBIA has outstanding insurance on about $673 billion of municipal and structured finance bonds, compared with about $14.3 billion of claims-paying ability as of Sept 30.

Robert Genader, chairman and chief executive of bond insurer Ambac Financial Group Inc, said last month if his company needed more capital it could write more insurance on bonds that require less capital, such as municipal bonds, and less insurance on securities requiring more capital, such as asset-backed securities.

Ambac could also write less new business in general, or use reinsurance, to increase capital levels, Genader said in a presentation on November 28.

MBIA said in a quarterly statement with regulators filed in November that capital raising was possible. The Company may undertake capital raising initiatives or other measures to raise or preserve capital, even in the absence of any immediate rating agency requirements, the filing said.

(Reporting by Dan Wilchins, editing by Gerald E. McCormick/Dave Zimmerman)