BOSTON/TORONTO (Reuters) - Billionaire investor William Ackman announced on Thursday that he sold 6 million shares in Canadian Pacific Railway Ltd (>> Canadian Pacific Railway Limited) after a huge run-up in the stock, adding a large pile of cash to his $11 billion hedge fund.

Just over a year after winning a proxy fight and installing a new chief executive at Canada's second-largest railway, the sale will add roughly $800 million to Ackman's Pershing Square Capital Management fund.

Even after the sale, Pershing Square will remain Canadian Pacific's biggest shareholder with a 10 percent stake and Ackman and Paul Hilal, a Pershing Square partner, will stay on the company's board.

Ackman said in June that he would sell 7 million shares after the stock's strong gains had ballooned the position in his fund.

While Ackman had projected the planned sales to take up to 12 months, calls from analysts and other investors to hasten the exit prompted Pershing Square to sell all at once in a public offering with the help of three Wall Street investment banks - Credit Suisse (>> Credit Suisse Group AG), Bank of America Merrill Lynch (>> Bank of America Corp) and Morgan Stanley (>> Morgan Stanley).

Analysts said some investors had expressed concerns that the share price, which has surged 13.62 percent in the last month alone, could be hurt by expectations of Pershing Square's trickling sales, so the move to sell fast made sense.

The hedge fund, which made a $2 billion profit on Canadian Pacific in less than two years, had already sold 1 million shares over the last four months.

A spokeswoman for Ackman declined to comment beyond the official filings.

Industry analysts and investors are wondering just how Ackman may use the growing pile of cash he is amassing. In late July, Ackman announced his last big bet, a $2.2 billion investment in Air Products and Chemicals Inc (>> Air Products & Chemicals, Inc.).

And in spite of lackluster performance at his fund through the summer, several big investors with Ackman said they had no plans to adjust their allocations to Pershing Square. Several investors said performance picked up in October.

For the fund, one of the biggest in the hedge fund industry, the Canadian Pacific marks the second big sale in less than two months. In August Ackman sold his entire position in J.C. Penney Co (>> J.C. Penney Company, Inc.), only days after resigning his seat on the company's board.

Ackman still has one position that is failing - the $1 billion short bet against nutrition company Herbalife Ltd (>> Herbalife Ltd.), which has cost him hundreds of millions in unrealized losses as the share price shot up 105 percent since January. Ackman has called the company a pyramid scheme, which Herbalife has denied. He recently said he restructured some of the bet.

Even as the stock price climbs, calls for regulators to investigate the company are growing louder. On Thursday a coalition, including a former consumer fraud litigator at the Wisconsin Department of Justice, urged the Federal Trade Commission to take enforcement action against "deceptive multi-level-marketing schemes."

Last week Latino civil rights groups appealed to California's attorney general to investigate the company, alleging that it improperly targets minorities with misleading advertisements. Herbalife has denied those claims and a spokesman for the attorney general would not confirm nor deny that the office was investigating.

(Reporting by Euan Rocha and Svea Herbst-Bayliss; Editing by Andre Grenon and Lisa Shumaker)

By Svea Herbst-Bayliss and Euan Rocha