Fitch effect: Buffett's Berkshire loses highest rating

Having legendary billionaire investor Warren Buffett as its "key man" and exposure to derivatives has cost the legendary investor-led Berkshire Hathaway the highest investment grade rating assigned to it.

Fitch has downgraded the company's rating from 'AAA' (highest investment grade) to a notch lower to 'AA+'.

This follows another rating agency Standard & Poor's cutting its 'AAA' rating on another blue-chip American entity, General Electric, which the world's largest industrial conglomerate had got in 1956.

This leaves just five American corporates -- ExxonMobil, Johnson & Johnson, Automated Data Processing (ADP), Microsoft and Pfizer -- with the top credit rating of 'AAA', down from more than 60 in early 1980s.

Even among these, S&P has warned about a possible downgrade of its rating on Pfizer, after the global pharma giant announced acquisition of rival Wyeth earlier this year.

Recently, Japanese auto giant Toyota, which has ousted General Motors as the world's biggest carmaker, also lost its AAA rating following a downgrade action by Moody's.

Fitch said that its ratings on Berkshire reflect the "long standing concerns with respect to key man risk in the form of the company's chairman chairman Buffett", but noted that the risk is unrelated to the legendary investor's age.