We may have another battle brewing between billionaire hedge-fund managers.
Leon Cooperman, chairman and CEO of Omega Advisors, told Bill Ackman not to interfere with ADP, the outspoken head of Pershing Square Capital's latest target.
"While I am always praising of you as a person and an intellect if I agree to say something it will be supportive of the company and critical of you. Just a heads up. I have no dog in this fight as I gave away all my stock to charity when I retired from the board (should have held the stock and give cash!), but this is a quality management that has done a great job over many years for the shareholders," Cooperman wrote in an email to Ackman obtained by CNBC's Scott Wapner.
"Their financial policies have always been balanced and intelligent, management is rationally compensated, they have spun out businesses showing no interest in building empires and they have a level of client satisfaction and retention second to none. The idea that you can tell these guys how to run their business doesn't strike me as intelligent or appropriate," he added.
Cooperman has served on the board of directors of ADP for two decades through 2012.
Ackman's Pershing is seeking control of ADP, including five board seats and the ouster of the chief executive Carlos Rodriguez, demands the payroll processing company has rejected, according to an ADP statement Friday.
Ackman has built an 8 percent stake in ADP, mostly in derivatives, the company said.
Word that Pershing Square was building a position in the company sent the shares up more than 10 percent late last month. Pershing previously owned ADP shares from 2009 to 2011.
Cooperman founded Omega Advisors in 1991. The firm has approximately $3.6 billion in assets under management, according to its website.
— Update: Bill Ackman told CNBC later on Friday he is not seeking control of ADP and plans to nominate a minority slate of directors.
— CNBC's Scott Wapner and Liz Moyer contributed to this story.