Former board member Bill Ackman is officially through with the retailer. J.C. Penney's biggest investor sold his entire 18% stake to Citigroup and took a loss of more than USD400 million on the deal. On Wednesday, Ackman disclosed in a regulatory filing that he sold 39.1 million shares to Citigroup for USD12.60 per share. That's almost half of the average USD25 a share he paid in 2010 when he first invested in Penney.

Ackman sold his shares just two weeks after he resigned from Penney's board. His resignation was part of a deal to resolve a public battle between the activist investor and the struggling department store operator.

Ackman released public statements indicating he had lost confidence in Penney's board. He also stated that Chairman Thomas Engibous should be replaced. Ackman also fought with the retailer's board over how soon the company's CEO, Mike Ullman, should be replaced.

J.C. Penney Co Inc is in the process of recovering from a failed transformation plan spearheaded by its former CEO Ron Johnson. Johnson was ousted in April after only 17 months on the job.