The Sentix research group said its investor sentiment index for the euro zone dropped to -13.7 in August from -5.8 in July, far below the -7.7 analysts had forecast.

Expectations fell to -20.0 from -13.0, their lowest since August 2012, when the euro zone debt crisis was in full swing.

European Central Bank President Mario Draghi promised in July 2012 to do "whatever it takes" to save the euro, a pledge that is widely credited with holding the bloc together.

"As the end of Draghi's time in office nears, economic expectations have again reached a point where Draghi felt obliged to give his 'whatever it takes' speech," Sentix director Patrick Hussy said.

In July the ECB revamped its interest rate guidance and asked its staff to prepare options for more policy easing, explicitly opening the door to a rate cut and more bond purchases as soon as September. The U.S. Federal Reserve cut interest rates last week .

"Measures announced by central banks have not turned economic expectations around," Hussy said.

A sub-index for Germany tumbled to -13.7, its weakest since August 2009, from -4.8. Sentix said dependence on exports and concern over China was a burden for Europe's largest economy, as a dispute over tariffs dragged on longer than many had hoped.

"A recession in Germany is preprogrammed," Hussy said.

Sentix said 930 investors took part in its survey, which was conducted from Aug. 1 to Aug. 3.