Ackman said he expects the outbreak, which has been detected in roughly 80 countries, to weigh on growth in the United States and abroad and affect both stock and credit markets.

Over the last 10 days, "we have taken steps to protect the portfolio from downward market volatility," wrote Ackman, who oversees Pershing Square Capital Management. He declined to say exactly what steps he took.

Ackman spoke out on the same day that the Federal Reserve announced an emergency interest rate cut. Ackman's statement, an unusual move for someone who seldom discusses portfolio movements publicly, underscores investors' fears that the coronavirus will have serious implications for economic growth.

"We believe that efforts to contain the coronavirus are likely to have a substantial negative impact on the U.S. and global economies, and on equity and credit markets," the statement said.

Ackman said hedging was preferable to selling off his portfolio of companies whose businesses are otherwise strong and include Chipotle Mexican Grill, Hilton Worldwide, home improvement chain Lowe's, Burger King operator Restaurant Brands International and Berkshire Hathaway, among others. The hedge fund sold out of its position in Starbucks earlier in the year.

In the first two months of the year, the firm's Pershing Square Holdings portfolio fell 7.1%, after having gained 58.1% last year, which made it one of the industry's best-performing hedge funds. The market drop has taken a bite out of the size of Ackman's funds, according to a report which put assets under management at $6.6 billion at the end of February compared with $7.5 billion at the end of December.

(Reporting by Svea Herbst-Bayliss; Editing by Leslie Adler)

By Svea Herbst-Bayliss