More than two-thirds of U.S. chief executives plan additional layoffs and expect sales to decline in the next six months as their confidence in the economy continues to fall, according to a survey released on Tuesday.

The Business Roundtable's quarterly CEO Economic Outlook Index fell to negative 5 -- the first negative reading in the survey's six-year history -- and down from a fourth-quarter reading of 16.5. A reading below 50 means CEOs expect contraction rather than growth.

"This quarter's results reflect the complex ongoing challenges currently at work in the U.S. economy," said Harold McGraw, who serves as Roundtable chairman and is also chief executive of publisher McGraw-Hill Cos. "Reduced consumer demand both here in the United States and abroad has placed significant pressure on American businesses."

One hundred U.S. CEOs were polled between March 16 and March 27 -- during the recent rally in U.S. stocks -- said they now expect real U.S. gross domestic product to decline 1.9 percent this year. That is below their December forecast, which anticipated flat GDP.