Spain today reported the steepest slide in consumer prices in nearly five years, a potentially worrying development as the euro zone fends off the threat of a deflationary spiral. 

With Spaniards cautious about spending in an economy suffering a 24% unemployment rate, consumer prices dropped a sharper than expected 0.4% in the year to July, Spain's National Statistics Institute said. 

The fall in prices in July was the sharpest since October 2009, the institute said, after revising its initial estimate of a 0.3% decline. 

In the previous month, Spain's economy, the fourth-largest in the euro zone, had reported an annual inflation rate of zero. 

Prices were kept down by the lower cost of entertainment, electricity, tobacco, food and soft drinks, the official statistician said. 

When compared to June, consumer prices slumped 1.5%, it said, using figures that are measured in the same way across the European Union. 

The Spanish inflation report is likely to be of concern to policymakers trying to safeguard a gradual recovery in activity since the economy emerged in in mid-2013 from a double-dip recession. 

Broad, sustained falls in consumer prices can lead people to postpone purchases in the hope of future price declines, a reaction that slows economic activity. The phenomenon can quickly degenerate into a vicious downward spiral that is notoriously difficult to reverse. 

The European Central Bank has taken unprecedented measures to head off deflation, including the introduction of negative interest rates for banks that want to deposit excess reserves with the central bank. 

Last week, the ECB held key its interest rates at a record low level, stimulating economic activity by making it cheaper to borrow money. 

ECB president Mario Draghi pledged to keep rates low for an "extended period" in view of the inflation outlook in the single currency bloc. "The governing council is unanimous in its commitment to also using unconventional instruments within its mandate, should it become necessary to further address risks of too prolonged a period of low inflation," he said.