The number of investors who think a country will leave the euro zone soon has dropped by two thirds in the year since the head of the bloc's central bank promised to safeguard the currency, a new survey shows.

The July poll of 888 investors by German research group Sentix showed 23.75% of participants foresaw at least one state quitting the 17-nation bloc within the next year.

Last July's euro break-up index reading was 73%.

That survey coincided with the pledge by European Central Bank President Mario Draghi to do "whatever it takes" to save the euro.

However, economic stresses in the euro zone remain significant. Sentix's most recent breakup reading rose slightly from June and the risk of contagion hit a record high.

ECB President Mario Draghi gave his message of support last July as Spain and Italy faced rising pressure on financial markets and Greece held fraught meetings with international lenders after failing to stick to economic targets set under its bailout.

His promise calmed investors and led six weeks later to the Outright Monetary Transactions (OMT) programme, under which the ECB conditionally offered to buy unlimited amounts of bonds from struggling states. The OMT programme has yet to be tapped.

A political crisis in Portugal and concerns about possible snap elections there drove the Sentix index on euro zone break-up around 4 percentage points on the month in July.

The percentage of investors expecting the country to leave the euro zone doubled from 2.66% in June to 5.38%. Sentix also said the risk of contagion hit an all-time peak of 43.5%.

"If investors were worried especially about Greece in July 2012, we have Cyprus since March 2013 and now, with Portugal, (there are) three states that hang in the balance should push come to shove," Sentix said in a statement.