FILE PHOTO: Billionaire activist-investor Carl Icahn gives an interview on FOX Business Network's Neil Cavuto show in New York, U.S., February 11, 2014. REUTERS/Brendan McDermid/File Photo

BOSTON (Reuters) - Billionaire investor Carl Icahn, who sparred for years with short-seller William Ackman over the future of Herbalife Nutrition Ltd HLF.N, on Thursday said he sold roughly $552 million worth of Herbalife stock back to the company, trimming his stake to 21 percent.

Icahn said in a regulatory filing that he sold 10.5 million shares back to the company, for $52.50 a share, after having announced last week that he was ready to part with 11.4 million shares.

The 82-year-old investor remains the nutrition and supplements company’s biggest owner after cutting his stake to 35.2 million shares from 45.7 million, which represented a 26.2 percent stake.

Herbalife shares were trading at $50.72, up 2.3 percent, on Thursday shortly before the market close.

Icahn said he was selling because the position had become too large within his portfolio. The sale comes roughly three months after Ackman ended his campaign against the company.

In late 2012, Ackman unveiled a $1 billion short position and predicted Herbalife’s share price would eventually fall to zero. Icahn, however, was convinced that the company’s future was bright and began buying. In February 2013 Icahn said in a filing that he paid an average $21.03 for his shares. He continued to buy Herbalife shares after that.

Since January, Herbalife’s shares have climbed 46.4 percent.