BOSTON (Reuters) - Billionaire investor William Ackman adjusted his battered hedge fund portfolio on Wednesday when he sold 20 million shares of snack maker Mondelez International (>> Mondelez International Inc), one day after another key holding, Valeant Pharmaceuticals (>> Valeant Pharmaceuticals Intl Inc), lost half of its value.

In a three paragraph letter sent to investors after the close of trading on Wednesday and seen by Reuters, Ackman said his firm, Pershing Square, has no plans to sell other investments right now and has "substantial uninvested cash."

The letter came one day after Pershing Square lost about $800 million when Valeant was pummelled after raising the possibility that it may default on its debt and posting a worse-than-expected financial outlook.

Ackman's Pershing Square Holdings has now lost 26.4 percent since January, marking one of the worst performances in the hedge fund industry this year.

Before today's sale, Pershing Square owned 43 million shares, or 7.5 percent, of Mondelez, which makes Oreo cookies, Trident gum and Milka chocolate. It now owns 5.6 percent of the company. The hedge fund unveiled its position in Mondelez in August 2015 and has since been pushing for a sale.

"We continue to believe in the potential for operating improvements and margin expansion that we expect will lead to substantial further increases in value," Ackman wrote in the letter.

On Tuesday, Ackman barely contained his frustration with Valeant in an earlier note, warning the company and assuring his investors that Pershing Square was "going to take a more proactive role at the company to protect and maximize the value of our investment."

Last week, Pershing Square's vice chairman, Steve Fraidin, a veteran mergers and acquisitions lawyer, joined Valeant's board.

It was roughly a year ago that Ackman unveiled his investment in Valeant, coming not quite a year after working with the fast-growing company on a deal in 2014 to buy Allergan, in which Pershing Square owned shares. The bet on Allergan helped Pershing Square earn a 40 percent gain in 2014.

The size of Ackman's fund has shrunk dramatically to roughly $12 billion at the end of February from about $19 billion in early 2015. But because he listed shares on the Amsterdam stock exchange in 2014, he has a cushion of permanent capital.

Still, investors have expressed concern about the fund's performance and Ackman urged them in Tuesday's and Wednesday's letters to call the investor relations team or him personally with any questions.

(Reporting by Svea Herbst-Bayliss; Editing by Sandra Maler and Alan Crosby)

By Svea Herbst-Bayliss