Steve Kulmar said he now regretted selling the agency to Y&R Brands, the WPP subsidiary, three years ago.
Kulmar, who plans to start a rival agency in July, said: "I don't see eye-to-eye with WPP. I'm an entrepreneur and WPP is a huge corporation."
He added: "WPP is heavily focused on profitability, which isn't suitable in the retail marketing space. This sector is about developing trusting relationships, not short-term profits."
Matt McGrath, the Y&R Brands chief executive, said: "I was aware of his frustrations, but Steve and I got on very well. There were no issues between us."
