Warren Buffett's investment company has lost its top credit rating, in part because of its reliance on its 78-year-old founder.

Fitch, the credit-rating agency, stripped Berkshire Hathaway of its AAA grade last night, citing risks stemming from derivatives holdings and Buffett's role as chief investment officer. The other two major credit-rating agencies, Standard & Poor's and Moody's, still rate Berkshire triple-A.

Buffett, the legendary US investor dubbed the Sage of Omaha, was knocked off top spot in the Forbes rich list this week by a resurgent Bill Gates. He recently had to tell investors in his company that 2008 had been the worst year since he took the helm 44 years ago.

Fitch said that Buffett's role as chief investment officer puts the company at risk if he becomes unable to do the job. The agency cut the issuer default rating on Berkshire to AA+ and senior unsecured debt to AA. The insurance and reinsurance units kept their AAA status, but Fitch talked of a negative outlook for all divisions.

"Fitch views this risk as unrelated to Mr Buffett's age, but rather Fitch's belief that Berkshire's record of outstanding long-term investment results and the company's ability to identify and purchase attractive operating companies is intimately tied to Mr Buffett," the agency said.

General Electric also suffered the indignity of seeing its credit rating downgraded yesterday. It was relegated to AA+ by Standard & Poor's, after holding the top AAA rating for more than 60 years. S&P said that GE was under "increasing earnings pressure, due to the recent sharp deterioration in general economic conditions around the globe".