Carl Icahn, billionaire investor and chairman of Icahn Enterprises Holdings LP, talks outside of the Nasdaq MarketSite in New York in 2012. (Scott Eells/Bloomberg News)

Billionaire and activist investor Carl Icahn on Thursday amped up his public campaign for changes at Apple, calling on the tech giant to buy back $150 billion worth of stock.

The letter to Apple’s chief executive, Tim Cook, was the latest example of Icahn’s efforts to persuade the firm to adopt new policies. He has sent tweets, including one noting that he and Cook had dinner.

In the letter to Cook on Thursday, Icahn said he had increased his investment in the tech giant by 22 percent, to 4.7 million shares, and predicted that a buyback would push the stock price up to $1,250 in three years. (It closed at about $530 a share Thursday, up about 1 percent, but is flat for the year.)

Icahn published the letter on his new Web site, Shareholders Square Table. The site, which has a picture of a shareholder mob storming a castle labeled “Your Company,” says its purpose is to provide a platform for investors.

The call for an Apple buyback appears to be the site’s first major project, one that takes up an argument that Icahn has repeatedly pushed.

On Oct. 1, Icahn wrote to his more than 100,000 Twitter followers that he was pushing Cook for a buyback while having a “cordial” dinner with the Apple chief executive.

In the letter to Cook, Icahn said that Apple could finance the buyback with debt or a mix of debt and cash. He argued that with $147 billion in cash, Apple would be in a good position to finance such a big deal.

“While this would certainly be unprecedented because of its size, it is actually appropriate and manageable relative to the size and financial strength of your company,” Icahn wrote.

Icahn also said that he would withhold his own shares from the proposed buyback to “invalidate any possible criticism” that he would profit from the deal.

“There is nothing short term about my intentions here,” he said.

Apple did not return a call for comment on the letter. The company has previously said it would return $100 million to shareholders by the end of 2015.

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