NEW YORK — The battle over J.C. Penney’s future has turned uglier.

Activist investor Bill Ackman said Friday he has lost confidence in J.C. Penney’s board and its chairman should be replaced.

The statement adds more fuel to an unusually public squabble between Ackman, whose company Pershing Square Capital has a nearly 18 percent stake in the company, and the rest of J.C. Penney’s board.

The disagreement is over how long it will take to find a new CEO for J.C. Penney. Ackman says the process should take 30 to 45 days. The board has said CEO Mike Ullman is the right person for the job for now.

In a lengthy letter to the board, Ackman complained that Chairman Thomas Engibous didn’t call a proper meeting to discuss his views that J.C. Penney’s search process should be speeded up. He also questioned the board’s hiring and firing practices and “aggressive” inventory purchases.

Ackman has suggested former CEO Allen Questrom should replace Engibous. Questrom said that he would rejoin the company but not under a hostile board situation.

Ullman, who had been Penney’s CEO from 2004 to 2011, took back the reins in April. He took over from Ron Johnson, who was supported by Ackman but was ousted after 17 months on the job after his radical makeover led to massive losses.