Warren Buffett's Berkshire Hathaway Inc may lose its AAA credit rating from Standard & Poor's within a year if the insurance and investment company's capital levels or value of its equity holdings face more downward pressure.

S&P late Tuesday revised its rating outlook for Berkshire to negative from stable. It said any downgrade would probably be only one notch, to AA-plus.

S&P rival Fitch Ratings on March 12 lowered its equivalent rating on Berkshire to AA-plus from AAA. Moody's Investors Service rates Berkshire Aaa with a stable outlook.

In 2008 profit at Omaha, Nebraska-based Berkshire fell 62 percent and its net worth fell 9.6 percent, the worst year since Buffett took over in 1965.

The declines stemmed mainly from paper losses on derivative contracts tied to stock market indexes. Berkshire's equity holdings, such as American Express Co and Wells Fargo & Co , have also suffered big declines in value.

If continued substantial deterioration in the equity markets hurts capital further, or if it appears that the insurance group will not be able to restore capital back to the 'AAA' level through earnings or through capital contributions from Berkshire's noninsurance operations or external sources, then we might lower the ratings, S&P analyst John Iten wrote.

Fitch said Berkshire's earnings stream and capital volatility were inconsistent with the stability required of a AAA company. It said Berkshire also has key man risk because its track record and ability to find companies to buy is intimately tied to the 78-year-old Buffett.

Berkshire owns close to 80 companies. About half of its results come from insurance businesses. Buffett is the world's second-richest person, Forbes magazine said this month.

Class A shares of Berkshire closed Tuesday at $88,500 on the New York Stock Exchange. They began the year at $96,600.

(Reporting by Jonathan Stempel; editing by John Wallace)