IRVINE, Calif.—Bill Ackman, the activist investor who heads Pershing Square Capital Management, stepped up the pressure on Allergan Monday when he called for a special meeting of Allergan (NYSE: AGN) shareholders that could ignite a proxy battle for control of the company. If the special meeting is held, Allergan shareholders could potentially vote out six unnamed directors and replace them with directors who would approve its $53.8 billion bid for Allergan made by Ackman’s firm, Pershing Square Capital Management and Valeant Pharmaceuticals International (NYSE: VRX and TSX: VRX).

In response, Allergan advised stockholders to refrain from taking action until its board makes a recommendation.

Valeant and Pershing Square, a hedge fund that owns 9.7 percent of Allergan, have reiterated their most recent proposal to pay $72 in cash and 0.83 share of Valeant stock for each share of Allergan stock, which is $23.70 higher than their original bid on April 22. In addition, Valeant would pay Allergan shareholders up to $25 a share based on a sales target of Darpin, a drug for treating age-related macular degeneration that is in the early stages of development. Valeant said it would invest up to $400 million and retain Allergan employees to develop the drug.