Ackman Questions Impartiality of Herbalife’s Auditor

William A. Ackman expanded his campaign against Herbalife on Wednesday, questioning the independence of its auditor, Pricewaterhouse Coopers, and warning of “serious accounting” issues at the company.

Mr. Ackman, the hedge fund billionaire who runs Pershing Square Capital Management, questioned whether PricewaterhouseCoopers has a conflict of interest because of nonauditing work it performed for Herbalife, the nutritional supplement company. He has accused the company of being a pyramid scheme and has made a $1 billion bet that its stock will fall.

In a two-page note addressed to Dennis M. Nally and Robert E. Moritz, the chairmen of PricewaterhouseCoopers, Mr. Ackman urged the auditor to “carefully study the issues that we have raised” as part of its audit review. Attached to the note, which was also sent to the Securities and Exchange Commission and members of Herbalife’s audit committee, were 50 pages outlining Mr. Ackman’s contention that Herbalife is a fraudulent company.

The letter also asks Mr. Nally and Mr. Moritz to “explain how PwC intends to overcome and resolve the appearance of impaired independence with respect to its in-progress and impending audit and review in light of nonaudit services performed by PwC and/or members of the PwC global network for Herbalife.”

Herbalife hired PricewaterhouseCoopers in May 2013 after its former auditor KPMG resigned amid revelations that a KPMG partner had leaked nonpublic information about Herbalife to a friend and stock trader.

In May, Herbalife disclosed in a filing that it had used PricewaterhouseCoopers for payroll and administrative services but added that an internal audit committee had determined the earlier services would not have any bearing on the auditor’s objectivity or impartiality.

The 50 pages of notes attached to Mr. Ackman’s letter are part of a three-hour presentation he gave at a Sohn Conference Foundation gathering in Manhattan in December.

At the event, Mr. Ackman disclosed he had taken an “enormous” short position in Herbalife’s stock. Since the presentation, Herbalife’s stock price has increased by more than 75 percent, to $66.51 a share from about $37.

The letter to PricewaterhouseCoopers is dated Aug. 29 and was first reported by The New York Post.

PricewaterhouseCoopers declined to comment. A representative for Herbalife could not be reached.