NEW YORK, Sept 11 (Reuters) - Billionaire William Ackman is close to signing up Allergan investors who combined own at least 35 percent of the Botox maker to call a special shareholder meeting, according to people familiar with the matter, giving the hedge fund manager a victory in his fight to acquire Allergan.

Ackman, whose Pershing Square Capital Management is Allergan's largest shareholder with a 9.7 percent stake, has teamed up with Canada's Valeant Pharmaceuticals International Inc to pursue a hostile takeover bid for Allergan.

The people asked not to be named because the matter is not public. Representatives for Pershing and Valeant could not be immediately reached for comment.

(Reporting by Soyoung Kim and Olivia Oran in New York; Editing by Chizu Nomiyama)