Sept 23 (Reuters) - Billionaire William Ackman-backed Pershing Square Capital Management said on Tuesday that it would sue botox-maker Allergan Inc if it went ahead with a deal to buy drugmaker Salix Pharmaceuticals Ltd.

Allergan is trying to seal a deal with Salix in order to stay independent and fend off a $53 billion hostile offer from Valeant Pharmaceuticals Inc . The Valeant offer is backed by Pershing Square - Allergan's largest shareholder with a 9.72 percent stake.

In a letter to Allergan's board on Tuesday, Ackman said Pershing Square would sue the company for breach of fiduciary duty if it resumed its attempts to buy Salix without a shareholder vote.

Reuters reported on Monday that Allergan had resumed talks to buy Salix.

Pershing Square has been threatening to sue Allergan for more than two months, with Ackman calling its attempts at acquisitions "desperate".

(Reporting by Anjali Rao Koppala in Bangalore; Editing by Simon Jennings)