Imperial Sugar Corp., the third- biggest U.S. sugar processor, said Lehman Brothers Holdings Inc. may sell its 30 percent stake in the company. The shares plunged 15 percent, the biggest drop in six months.

About 3.3 million shares, acquired by Lehman as part of the sugar company’s reorganization in bankruptcy court, may be sold, Sugar Land, Texas-based Imperial said today in a filing with the U.S. Securities and Exchange Commission. Chief Executive Officer Robert Peiser said he expects Lehman to sell some of its Imperial shares, though not all.

“Just the fact they are thinking of selling, I’m not surprised that should put some pressure on the stock,” Peiser told analysts and investors on a conference call. “It is prudent that they lighten their investment.”

Lehman’s move comes after Imperial shares more than doubled this year. The share sale should benefit investors in the long term by creating greater trading liquidity, Peiser said. Lehman, the fourth-biggest U.S. securities firm, has not said when or how many shares may be sold, he said. Lehman spokeswoman Kerri Cohen declined to comment.

Shares of Imperial fell $5.36 to $30.23 at 4 p.m. in Nasdaq stock market trading, the biggest decline since Oct. 28.

Imperial sells refined sugar under brands such as Dixie Crystals and Holly to food manufacturers, retail grocers and foodservice distributors. American Sugar Refining Inc.’s Domino Sugar and United Sugars, a group of cooperatives, are the two biggest U.S. sugar refiners, Peiser said on the call.

Lehman Brothers did not register 39,861 shares, or 1.2 percent of the securities firm’s total, which were granted to managing director John K. Sweeney in his capacity as an Imperial director, the sugar company said.