The move comes at an awkward time for Unilever, the consumer-goods giant, which has been courting investors as it prepares to list its ice cream unit as a standalone firm in mid-November. The Financial Times first reported Greenfield's letter and plans.

Ben & Jerry's, famous for popular flavors including Chunky Monkey and Cherry Garcia, has long been outspoken on social and political issues. When Unilever bought the brand in 2000, Ben & Jerry's negotiated a detailed merger agreement to protect its right to speak out, which included the establishment of an independent board to oversee its social mission.

In recent years, the relationship between Unilever and Ben & Jerry's independent board has soured and the board is currently taking Unilever to court in the US over what it says are attempts to clamp down on its freedom.

"It's profoundly disappointing to come to the conclusion that that independence, the very basis of our sale to Unilever, is gone," Greenfield said in the letter.

Greenfield is stepping away from the position of brand ambassador, a salaried role he has held since the acquisition. He isn't involved in the legal dispute, Unilever said.

"We disagree with his [Jerry's] perspective and have sought to engage both co-founders in a constructive conversation on how to strengthen Ben & Jerry's powerful values-based position in the world," a spokesperson for Magnum said by email.

Greenfield and co-founder Ben Cohen have said they want to buy the brand back. Cohen told Bloomberg this month there's a group of investors that is "capable and ready" to make an offer for Ben & Jerry's. "The only problem is that Unilever and Magnum have stated that they're not open to that."

Cohen protested outside the London venue where the new unit, now called The Magnum Ice Cream Company, held its first capital markets day last week.

Ben & Jerry's founder quits, says Unilever 'silenced' activism - The Boston Globe