Billionaire U.S. investor Warren Buffett has lifted his stake in Tesco in a move likely to be seen as a vote of confidence in Chief Executive Phil Clarke after a profit warning from the world's No.3 retailer last week.

Buffett's Berkshire Hathaway Inc had increased its stake in the British supermarket group from 3.21 percent to 5.08 percent as of January 13, a regulatory filing showed on Thursday.

That's a day after Tesco warned trading profit for its 2012-13 financial year would be flat as it steps up investment in its home market following its worst underlying Christmas sales performance for decades.

Dubbed the Sage of Omaha for a string of investments that have propelled him to number three on Forbes' 2011 list of the world's wealthiest men, Buffett's share dealings are closely watched in financial markets.

After buying into Tesco in 2006, Buffett has gradually increased his holding.

Last year he caused a stir by saying Tesco should look hard at its loss-making Fresh & Easy chain in the United States, though he also said he remained supportive of the business.

Tesco shares dropped as much as 19 percent on January 12 and hit a 34-month low of 311 pence on Monday.

At 1255 GMT, the stock was up 0.5 percent at 322.5 pence.

Tesco has come under fire from some investors after news emerged that UK chief operating officer Noel Robbins sold around 200,000 pounds ($308,000) worth of shares eight days before its profit warning.

(Reporting by Mark Potter. Editing by Jane Merriman and David Cowell)