Microsoft Chief Executive Steve Ballmer has sold $1.3 billion in Microsoft stock over the past three days, and says he could sell $700 million more before the end of the year.

On Wednesday, Thursday and Friday, Ballmer sold 49 million shares, when the shares ranged between $26.74 and $27.16, according to a filing with the Securities and Exchange Commission.

He said Friday in a company statement that he plans to sell a total of 75 million shares by the end of the year. He said he is selling the stock to diversify his holdings and to help with tax planning. “Even though this is a personal financial matter, I want to be clear about this to avoid any confusion,” Ballmer said in the statement. “I am excited about our new products and the potential for our technology to change people’s lives, and I remain fully committed to Microsoft and its success.”

Microsoft declined to comment further on the stock sale.

Selling shares from his Microsoft holdings is unusual for Ballmer; the last time he did so was seven years ago, when he sold about $1 billion worth of stock.

If he does ultimately sell 75 million shares, that would represent 18 percent of his holdings and take his ownership stake in the company down to 3.9 percent. Until this week’s sales, he owned 408 million shares of Microsoft.

Ballmer remains the second largest individual shareholder. The largest is Chairman Bill Gates, whose 621 million shares represents 7.2 percent of the company, according to the Microsoft proxy statement filed in September.

Ballmer made $1.34 million in total compensation in fiscal 2010.

Microsoft stock has been languishing over the past several years. In the past 12 months, the stock price has ranged from $22.73 to $31.58.