Mr Buffett, a stock market guru and one of the world's richest men, said he would soon be forced by UK trading rules to make an announcement because his stake was approaching three per cent.
In a rare public appearance in London, Mr Buffett said he started buying the mystery company two months ago. He declined say when his stake would pass the three per cent mark. "It depends on what the price does," he said.
The US billionaire also refused to say whether the company - like many of his other investments - sold consumer products. But Mr Buffett, a one-time investor in Guinness, did hint that the company was a large UK blue chip. He said: "We do have a certain-sized requirement for investments. We need elephants now."
Mr Buffett who was promoting NetJets, his airline timeshare business, denied he was stalking the UK company. The shares had been bought by Berkshire Hathaway, his Nebraska-based investment vehicle. He said Berkshire Hathaway had $15bn in cash earmarked for investment, but high valuations of most stocks meant buying opportunities were thin on the ground.
He said: "If we're sitting on that kind of money it means we're having a hard time finding things to buy. If we find a good business run by honest and able businessmen we will buy it. If not we will wait."
The Berkshire Hathaway chairman declined to comment on the overall direction of US stocks or on particular companies in his investment portfolio.
Mr Buffett reaffirmed his commitment to two of his larger shareholdings, Coca Cola and Gillette, whose shares have been hit by earnings worries.
Mr Buffett brushed aside the global slowdown. "The macroeconomy makes no difference to us. I don't read economic forecasts. I don't read the funny papers. I simply try to figure out where a business will be 10 years from now."