Shares of Bermuda-based bond insurer Assured Guaranty fell 6.3 percent yesterday on concerns that its financial strength rating could be downgraded on new criteria proposed by ratings agency Standard & Poor’s.S&P said on Monday that it came up with nine categories to consider before deciding on a bond insurer’s rating: industry risk competitive position, management and strategy, enterprise risk management, operating performance, investments, liquidity, capital adequacy and financial flexibility.The rating agency said it could lower ratings on bond insurers if the proposed criteria are adopted unless those bond insurers raise more capital or reduce risk.“In particular, the amount of capital needed to achieve high investment-grade ratings will increase significantly under the proposed criteria because of higher capital charges used in scoring capital adequacy” and a new leverage test, S&P said.Assured Guaranty was the only bond insurer to survive the global financial crisis with its AAA credit rating intact because it avoided straying into the insurance of collateralised debt obligations and mortgage linked securities, each of which plummeted in value as property prices slumped and credit markets froze up.The top credit rating was an attribute that enabled Assured to virtually monopolise the market for insuring newly issued muncipal bonds in the US, and third-quarter operating income last year soared 382 percent from the prior year period to $222.8 million.But last October S&P downgraded the company to AA+ with a stable outlook, signalling the demise of the last AAA rating left standing in the bond insurance industry.Assured’s shares plunged more than 10 percent during trading on the New York Stock Exchange yesterday, but recovered late in the day to close down $1.01 at $15.06.Assured’s shares have plunged 23 percent since closing at $19.55 on January 14. The company has been backed to the tune of $1 billion by billionaire investor Wilbur Ross.