WASHINGTON — US consumer confidence plunged to the lowest level in more than three decades in early August, according to data published Friday by Reuters and the University of Michigan.

Their index of consumer sentiment tumbled to 54.9, its lowest reading since May 1980, when the index sank to 51.7.

The decline was much sharper than expected, and followed an equally sharp fall in July, to 63.7 from 71.5 in June.

The average analyst forecast for the August number had been 62.5.

The survey was taken during a period of high volatility in markets and when US politicians were battling over the country's debt and deficit-paring plan.

"Most of the hit was in the expectations index, which is most sensitive to stocks," said Ian Shepherdson of High Frequency Economics.

"This is grim, obviously, but remember that GDP is a measure of spending, not sentiment... If the index were to remain at this level, history would tell us to look for slower spending, but one month is not a trend."