Activist investor Carl Icahn said he will pursue a shareholder vote to force Apple Inc. to boost its stock buyback plan, stepping up his campaign to unlock more of the cash held by the world’s most valuable company.

The precatory, or non-binding, proposal will be for less than the $150 billion he had been advocating, Icahn said in a post on Twitter. He will propose a $50 billion buyback, CNBC reported.

Icahn, who has become a billionaire by buying stakes in companies and then publicly pushing for changes to boost the stock, has been urging Apple since August to increase the size of its buyback — even though the company earlier this year initiated one of the biggest programs among U.S. companies. Icahn disclosed in a filing today that he owns 4.7 million Apple shares, or about 0.5 percent of the iPhone maker.

“Apple is not a bank,” Icahn said in an interview with Time Magazine.

Icahn said in the interview that he filed the proposal on Nov. 26, in time to be considered at Apple’s next shareholder meeting. The measure won’t be binding to management even if approved by shareholders, he said in the interview.

Icahn told Time that his proposal isn’t an indictment of Apple Chief Executive Officer Tim Cook, who he said is “good whether he does what I want or not.” Icahn has said he and Cook have had dinner to discuss the matter.

Proxy Fight

“Earlier this year we more than doubled our capital return program to $100 billion, including the largest share repurchase authorization in history,” said Kristin Huguet, a spokeswoman for Apple. “As part of our regular review process, we are once again actively seeking our shareholders’ input on our program, and as we said in October, the management team and our board are engaged in an ongoing discussion about it which is thoughtful and deliberate. We will announce any changes to our current program in the first part of calendar 2014.”

Icahn didn’t immediately respond to a call for comment.

Apple has almost $150 billion in cash and investments. After years of spurning investor calls to implement a dividend or buyback, Apple has changed tack. It has returned $36 billion to shareholders since 2012, the company said in October.

By putting the measure to a shareholder vote, Icahn is starting a potential proxy fight with Apple. The investor could withdraw his plan if a deal is reached.

Icahn isn’t the first activist investor to urge Cupertino, California-based Apple to return more money to shareholders. Hedge-fund manager David Einhorn also has pressured Cook to use more of the company’s cash to reward investors.

Since Icahn disclosed on Aug. 13 that he owns Apple shares, the stock has increased 15 percent. Apple fell less than 1 percent to $565 today at the close in New York.