Standard & Poor's on Thursday stripped Warren Buffett's Berkshire Hathaway Inc. of its top AAA rating, saying its acquisition of Burlington Northern Santa Fe will hurt liquidity and capital adequacy.

The rating agency cut Berkshire's long-term counterparty credit rating rating by one notch to AA-plus. The outlook is stable, which typically means that S&P does not expect another rating action over the next two years.

"We believe that the railroad acquisition will reduce what historically has been extremely strong capital adequacy and liquidity, and that investment risk with sizable concentrations remains very high," S&P said in a statement.

Berkshire is buying Burlington, the second-largest U.S. railroad company, for roughly $26 billion in stock and cash.

"A key concern is that Berkshire's risk tolerances appear to have increased, yet we believe they remain ill-defined while the organization increases in complexity," S&P said.