Automatic Data Processing Inc. said activist investor Pershing Square Capital Management LP is "seeking control" of the company, and is attempting to shake up its board and push out CEO Carlos Rodriguez.

Pershing Square's William Ackman recently notified ADP of the hedge fund's 8% beneficial ownership in the company. He also said he plans to nominate five directors, including himself, to ADP's 10-member board.

ADP has rejected Ackman's demand for extending the Aug. 10 deadline for nomination of directors by 30 to 45 days. The company said in a statement that while its board "respects" shareholders' rights to nominate directors, it has "determined that it is not in the best interests of ADP or its other shareholders to accede to Pershing Square's last-minute request for an extension."

ADP in its statement also defended the company's current board composition and Rodriguez's tenure as CEO.

"Since Carlos Rodriguez became CEO nearly six years ago, ADP's total shareholder return of 202% is well in excess of the S&P 500 [total share holder returns] of 128% -- and is many multiples of Pershing's [total shareholder returns] of 29%," the company said.

Morgan Stanley & Co. is acting as financial adviser and Paul Weiss Rifkind Wharton & Garrison LLP is acting as legal adviser to ADP.