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Starbucks executive chairman Howard Schultz, who oversaw the growth of the coffee chain into a global powerhouse, has said he is quitting the company after nearly four decades.

Mr Schultz, who retired as chief executive last year, is to relinquish his seat on the board on 26 June.

The move has raised questions about his political ambitions.

In an interview with the New York Times, Mr Schultz did not rule out a run for president.

"One of the things I want to do in my next chapter is to figure out if there is a role I can play in giving back," he said.

"I'm not exactly sure what that means yet."

He added: "I want to be of service to our country, but that doesn't mean I need to run for public office to accomplish that."

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Social debates

Mr Schultz, 64, started working for Starbucks in 1982 as director of operations and marketing. He has led the board since 1985 and was chief executive for much of that period.

During his tenure with the firm, Starbucks grew from 11 outlets to more than 28,000 and its share price rose by 21,000%, the company said.

Mr Schultz, who will become chairman emeritus, also used his position to speak out in social debates.

Starbucks was one of the first US retailers to offer health insurance to its workers. It has also taken positions on issues such as immigration.

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Mr Schultz also offered a harsh assessment of his own firm last month, after Starbucks called police to report two black customers waiting for a friend, leading to their arrest for trespassing.

"I think what occurred was reprehensible at every single level," he told CBS News.

Starbucks said Mr Schultz planned to write a book and spend time with his family this summer.