Warren Buffett's Berkshire Hathaway has lost its last remaining triple-A credit rating.
Standard & Poor's dropped its ratings on Berkshire and its core subsidiaries from the top-level AAA to AA+.
S&P says Berkshire's planned acquisition of Burlington Northern Santa Fe prompted the move.
In a statement posted on its web site, S&P says "We believe that the railroad acquisition will reduce what historically has been extremely strong capital adequacy and liquidity," as Berkshire uses cash and borrowing to help finance the $26 billion deal.
S&P adds: "Investment risk remains very high in our view, compounding the need for extremely strong capital and liquidity given potential investment volatility. A key concern is that BRK's risk tolerances appear to have increased, yet we believe they remain ill defined while the organization increases in complexity."
Another concern, says S&P, is "uncertainty" surrounding who will be running Berkshire Hathaway after "the eventual transition of the company's leadership" from Warren Buffett.