The wealthy county of Oakland would lose its triple-A rating simply because its neighbours are in trouble.
For this reason, he told BBC One's Andrew Marr Show, the UK's triple-A rating was "hugely important".
Moody's had already cut Spain's sovereign debt rating from the top, triple-A rating to Aa1 in September.
Another agency, Fitch, confirmed France's triple-A rating on Friday evening, but revised its long-term outlook to "negative" from "stable".
The Fitch credit agency has taken away its triple-A rating, a veritable death knell for a bond insurer.
Fitch Ratings, which affirmed MBIA's triple-A rating on Wednesday, is still reviewing Ambac.
And many investors say they are braced for France to lose its triple-A rating in the early days of 2012.
It said it "could only conclude" that the goal posts had been moved, meaning Moody's requirements for a triple-A rating had changed.
The triple-A rating is important because it allows the company to continue to attract new business, particularly in the municipal bond market.
Without a triple-A rating to secure the insurance, municipalities would have a harder time raising money and would have to pay more to do so.
For instance, the fund has sharply cut its exposure to UK Treasury bonds, and this was before the downgrade in Britain's triple-A rating.
Ratings agencies have said they would consider downgrading the U.S. triple-A rating, the highest possible, if the government defaults on its current debt.
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Of the major economies, only Canada and Germany retain triple-A rating.
Chief amongst them is George Osborne, who used to treat protection of triple-A rating as a key target and measure of his economic competence and political virility.
It does, however, offer some reassurance for the rest of the UK, saying that it ought to keep its triple-A rating in the event of Scotland going its own way.
It did not change the UK's triple-A rating, but said it was at risk without a credible plan to put its debts on a "secure downward trajectory" by the next government.
Fitch Ratings on Friday lowered its outlook on France's triple-A rating to "negative" from "stable" and warned that it is considering downgrading the debt of six of the countries in the euro zone.
And while Fitch says the rest of the UK would keep its triple-A rating intact, it adds that the problems in the UK economy put it on negative watch - that is, at risk of a downgrade.
Great Britain, with high deficits, mounting debt, and a deficit-reduction plan that just might not work, retains its triple-A rating because it has its own currency and the rating companies increasingly consider governance when deciding whether to downgrade.
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In what could be an encouraging sign for sterling, some analysts noted that the euro hasn't been battered by Moody's decision last week to strip France of its triple-A rating, a move that was widely anticipated by investors.
First, the Economist magazine ran a much-publicised supplement warning of a "time bomb" unless France reformed fast, and now Moody's has downgraded French debt from its triple-A rating, citing the country's lack of growth and loss of competitiveness.
Moody's, the credit-rating agency, got publicity--for once, not negative--in January when it warned that the U.S. government may lose its triple-A rating within a decade if Uncle Sam doesn't do something drastic about the unfunded liabilities of Social Security, Medicare and Medicaid.
What's really more likely to happen is that if these bond insurers get down-graded by the rating agencies from their triple-A rating, that would cause all the holders of the bonds to also have down-grades, which would mean yet more billions of write-offs, yet more losses, yet more problems for the stock market and yet more ripple effect on the rest of the economy.
So, we could still be looking at the US losing its triple-A credit rating.
German bonds reflect the country's strong economy, triple-A credit rating and ease of trading or liquidity in the market.
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Investors now are questioning whether Holland will lose its prized triple-A credit rating.
The pound has been weakening this year following some disappointing economic data and worries that the UK may lose its triple-A credit rating.
France could be stripped of its triple-A credit rating before Christmas, raising new doubts about the survival of the euro, analysts have predicted.
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Britain's triple-A credit rating seems assured, unlike France's (see article).
While Democrats and Republicans fritter away the time politically posturing and holding tight to their party lines, the U.S. comes closer to losing its coveted Triple-A credit rating.
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