Herbalife Ltd.'s biggest critic, hedge fund manager Bill Ackman, said he had been approached to buy shares in the nutritional supplements maker from billionaire investor Carl Icahn.

Ackman, who has bet more than $1 billion against Herbalife, said on CNBC that Icahn's planned stake sale would accelerate the company's downfall.

"This is a confidence game. Carl is what creates the confidence in the company. With Carl exiting, I think the thing is over, and over quickly. The sooner he sells the better," Ackman said. "I think he knows that this thing is toast."

Herbalife's shares slumped 7 percent in early trading on Friday.

Icahn and Ackman have opposing bets on Los Angeles-based Herbalife. Ackman for years has accused Herbalife of running a pyramid scheme and bet that the stock would fall to zero.

The duo even became embroiled in a public war of words, with Icahn famously calling Ackman a "liar" and a "crybaby" in a CNBC interview in 2013. They have since made up.

While Ackman has lost money on his short bet over the years, Icahn has made money but his gains are now at risk of shrinking as the stock has fallen in the last few days.

The approach to Ackman comes a little over a month after the company agreed to pay $200 million and change the way it does business to avoid being labeled a pyramid scheme by U.S. regulators.

Investment bank Jefferies Group has been trying to find buyers for Icahn's 18.3 percent stake for about a month, the Wall Street Journal reported on Friday. (http://on.wsj.com/2bMkIA0)

Sources told Reuters on Thursday that Icahn was considering structuring a sale of Herbalife shares.

Ackman's Pershing Square declined to comment. Jefferies and Herbalife were not immediately available for comment. Icahn was not immediately available for comment.

Following the settlement with regulators in July, Herbalife said its board had cleared the way for Icahn to boost his stake in the company to as much as 35 percent.

Since then, the company's shares have risen 4.3 percent.