May 21 (Bloomberg) -- Bill Gross, the co-chief investment officer of Pacific Investment Management Co., said the U.S. “eventually” will lose its AAA rating, but not any time soon.

“It’s certainly nothing that’s going to happen overnight,” Newport Beach, California-based Gross said in an interview today on Bloomberg Television. “The markets are beginning to anticipate the possibility.”

Standard & Poor’s lowered its outlook on the U.K.’s AAA credit rating today to “negative” from “stable” and said the nation faces a one-in-three chance of a rating cut as its debt approaches 100 percent of gross domestic product. U.S. marketable debt is at about 45 percent of GDP, according to Bloomberg data.

A downgrade was not “imminent,” Gross earlier told CNBC after Reuters reported he said via e-mail the U.S. dollar, equities and government bonds fell on speculation the nation is at risk of losing its top credit rating.

The administration of President Barack Obama has pushed the nation’s marketable debt to an unprecedented $6.36 trillion and raised on May 11 its estimate for the deficit this year to a record $1.84 trillion, up 5 percent from the February estimate, and equal to about 13 percent of the nation’s GDP.

U.K., U.S. Debt

“Both the U.K. and the U.S. have prospective deficits of 10 percent annually as far as the eye can see,” Gross said. “At some point over the next several years” the debt of each “may approach 100 percent of GDP, which is a level at which country downgrades tend to occur,” he said.

The dollar dropped to a four-month low against the euro, the Standard & Poor’s 500 Index declined 1.7 percent and the 10- year Treasury yield rose 0.17 percentage point to 3.37 percent.

The U.S. will issue a record $3.25 trillion of debt in the fiscal year ending Sept. 30, according to Goldman Sachs Group Inc., one of the 16 primary dealers that trade directly with the Fed and are required to participate in Treasury auctions.

“The market knows and believes that both the U.S. and the U.K. are quite similar in terms of their debt levels and debt trends,” Gross said.

Gross’s comments today come two months after he said the U.S. government will need to spend as much as $4 trillion in additional capital to cushion a slowing economy. The Federal Reserve said March 19 that it would purchase $1.8 trillion in Treasuries and housing-related debt to lower borrowing costs.

“We need more than that,” Gross said at the time. The Fed’s balance sheet “will probably have to grow to about $5 trillion or $6 trillion,” he said.

The Fed’s balance sheet currently stands at $2.2 trillion through the week ended May 13, up from $906 billion in the week ended Sept. 3.

To contact the reporter on this story: Dakin Campbell in New York at dcampbell27@bloomberg.net

Last Updated: May 21, 2009 16:35 EDT