Richard Schaefer has stepped down as chief executive of Oscar de la Hoya's Golden Boy Promotions.
Schaefer and De la Hoya had been at odds over the direction of the company, although De la Hoya insisted last month that he wanted him to stay.
However, Schaefer, a Swiss national who left banking to join De la Hoya's fledgling promotional company more than 10 years ago, said he felt compelled to leave.
Tension between the pair is reported to stem from De la Hoya’s decision to reach out to rival promoter Bob Arum of Top Rank.
They have not promoted together in years, a situation that scuppered a series of potentially lucrative fights.
"This decision has required a great deal of personal reflection, but ultimately I concluded that I have no choice but to leave," said Schaefer.
"I am proud to remain a shareholder, so I have a strong interest in the continued success of the company."
The news comes just days before Olympic gold medallist turned multiple professional world champion De la Hoya's scheduled induction into the International Boxing Hall of Fame in Canastota, New York, on Sunday.