Activist investor Bill Ackman urged Botox maker Allergan Inc (AGN) to negotiate a possible takeover by Valeant Pharmaceuticals Inc and not delay a special shareholder meeting that could overturn most of Allergan's board.

Ackman's Pershing Square Capital Management hedge fund, Allergan's largest shareholder with a 9.7 percent stake, is attempting to gain support of shareholders owning 25 percent of Allergan to call a special meeting, during which it would seek to remove six directors.

Allergan can delay the meeting until as late as November.

"We, on behalf of Allergan's other shareholders, ask that you do not delay the inevitable any further," Ackman wrote in a letter to Allergan's board, released on Wednesday. "What legitimate board of directors attempts to silence or otherwise delay hearing what its own shareholders have to say?"

Ackman also called Allergan's criticism of Valeant a "scorched earth response" and said spreading false information to drive down Valeant's stock would be market manipulation.

Allergan, known for its Botox anti-wrinkle injections, did not immediately comment on the letter.

Laval, Quebec-based Valeant, made an unsolicited cash and stock bid for California-based Allergan in April and has since raised its $51-billion offer.

Allergan shares rose 0.2 percent at $164.71 in early trading in New York, while Valeant stock added 0.4 percent to $119.93.