In an interview, Buffet, who owns 9 percent of Kraft's stock, said he had "a lot of doubts," about the deal to which the two firms agreed earlier this week and would have voted against it if given the chance, The Financial Times reported Wednesday.
Buffet said Kraft is currently undervalued and Kraft stock traded for Cadbury stock in the deal was "very expensive currency."
Kraft said: "We respect Buffet's opinion. He's one of our largest investors."
However, "we think this is a good deal for us. It transforms our portfolio for better long term growth."
Buffet called Kraft Chairman and Chief Executive Officer Irene Rosenfeld "a good operator," but said the deal to sell its frozen pizza business to Nestle for $3.7 billion to raise cash for the Cadbury deal was struck "in an enormously tax-inefficient way."
On Wednesday, credit rating agency Fitch lowered Kraft's rating to BBB minus due to "the anticipated increase in financial leverage of the combined Kraft/Cadbury."
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