Herbalife CEO Michael Johnson sharply denied Ackman's allegations.

"This is blatant market manipulation," Johnson told CNBC in a telephone interview Wednesday. "This appears to be another attempt to legally manipulate the market by a group of short sellers."

Johnson added: "This is not about Herbalife's business model. This is about Bill Ackman's business model."



"We're not a pyramid scheme, that's a bogus accusation," Johnson added. "We have millions of customers around the world. We don't pay for recruiting. We've been in business for 32 years."



(Read More: Herbalife Under Fire: Clubs, Hopes and Losses .)



A pyramid scheme is an illegal investment scam where contributions from new investors are used to pay off the original investors.

Ackman will provide more details on his short thesis as part of a two-hour presentation at the Sohn Conference Foundation investment gathering Thursday morning in midtown Manhattan, according to people familiar with the agenda.



(Read More: Selling the American Dream: Investigations Inc.)



Herbalife stock has been battered this year amid sharp questions about its business model from a variety of sources, including other hedge-fund managers.

A Belgium court recently ruled that Herbalife was an illegal pyramid scheme. The company, however believes its marketing plan was misunderstood and is appealing the ruling.

(Read More: Herbalife CEO: We're Not a Pyramid Scheme)

