Activist investor Daniel Loeb's Third Point LLC raised its stake in Sotheby's and called for the resignation of the auctioneer's chief executive, citing concerns about the company's leadership and strategic direction.

The hedge fund, now Sotheby's largest shareholder with a 9.3 percent stake, said it would look for a new CEO after joining the company's board.

"We are troubled by the company's chronically weak operating margins and deteriorating competitive position relative to Christie's, as evidenced by each of the contemporary and modern art evening sales over the last several years," Third Point said in a regulatory filing.

Sotheby's was not immediately available for comment.

Third Point also called for the positions of CEO and chairman to be separated. William Ruprecht currently holds both positions.

Last month Sotheby's said it would review its capital allocation strategy after Loeb and Nelson Peltz revealed big stakes in the company.

Sotheby's shares were up 0.5 percent at $49.98 in early trading on the New York Stock Exchange.