William Ackman, one of the world's biggest hedge fund managers, said on Wednesday that he is shorting Herbalife Ltd, a move which helped send the weight management products company's stock price down more than 12 percent.

Ackman, who oversees $11 billion in assets at Pershing Square Capital Management, confirmed that he was betting against Herbalife when asked about the matter on Wednesday.

Ackman is expected to spell out his reasons for the move at a hastily arranged investment conference in New York on Thursday.

Traditionally Pershing Square has been known to take long positions in companies and so a short position is particularly noteworthy.