By Paul Hannon 

Europe's struggle to avoid a slide into deflation suffered a setback in November, as consumer prices across the European Union's 28 members rose at the slowest annual pace in five years.

The bloc's statistics agency on Wednesday confirmed that consumer prices in the 18 countries that share the euro were just 0.3% higher than in the same month of 2013, while they were 0.4% higher in the EU as a whole. In October, the annual rates of inflation were 0.4% and 0.5%, respectively.

In both the eurozone and the EU, that returned the inflation rate to September lows that hadn't been matched since October and September of 2009 respectively.

With oil prices tumbling, inflation rates around the continent appear likely to decline further in coming months, and may turn negative in the early months of next year as prices fall below their levels of a year earlier.

Economists at Citibank lowered their near-term inflation forecasts on Wednesday and now expect prices in December to be 0.1% lower than a year earlier, with prices in January down 0.2%. The last time prices were lower than a year earlier was in September 2009, following the sharp economic contraction that followed the onset of the financial crisis in late 2008.

The European Central Bank has said it will reassess its existing stimulus policies, which include cheap bank loans and purchases of asset-backed securities and covered bonds, in early 2015, and decide whether to do more to ensure that annual inflation moves closer to its target of just below 2%.

Other central banks across Europe face similar challenges. Sweden's Riksbank on Tuesday listed a range of options it would consider if the outlook for inflation were to continue to worsen, including asset buying, lending to banks, a negative main rate and currency market interventions.

In an interview with The Wall Street Journal on Monday, Polish rate-setter Jerzy Hausner said the central bank should consider cutting its benchmark interest rate again if prices continued to fall early next year.

The problem of low inflation is now being felt even in the U.K., which typically has higher inflation rates than elsewhere.

Its measure of inflation fell to a 12-year low of 1% in November, and Bank of England Governor Mark Carney acknowledged it will fall further, requiring him to write a letter to the government explaining why the central bank had failed to meet its inflation target.

When inflation rates were higher, policy makers would likely have ignored falling oil prices, expecting the boost to real household spending power to compensate over the medium term as prices of other goods and services rose.

But the very low levels of inflation across Europe can be harmful to economic growth, crimping company profits and investment, while making it more difficult for governments and households to reduce high levels of debt.

And a slide into deflation would exacerbate those problems. Policy makers worry that if consumer prices start to fall, businesses and households will start to postpone spending decisions in the expectation that goods will be cheaper in the future. That in turn can become a self-fulfilling spiral, and the case of Japan has shown how difficult it can be to revive growth after deflation has taken hold.

"While the ECB would normally look through any drops in the headline eurozone inflation rate resulting from falling oil prices, the bank will be seriously concerned that this will lead to a further weakening in inflation expectations that then feeds through to result in a further drop in already worryingly low core inflation," said Howard Archer, an economist at IHS.

Still, some key policy makers believe the risk of a slide into a prolonged period of deflation remains very low.

"For me, a few months of inflation rates below zero does not constitute deflation. That would require a self-perpetuating downward spiral of negative inflation rates, GDP declines and wage declines," Jens Weidmann, president of Germany's Bundesbank and a member of the ECB's governing council, told reporters Monday. "This risk remains minimal."

Separate figures released by Eurostat show wage rises remained at modest levels in the three months to September, and are therefore unlikely to contribute to a pickup in inflation.

Total hourly labor costs were up 1.3% from the third quarter of 2013, a slight slowdown from the 1.4% rate of increase recorded in the second quarter. Wage rises were unchanged at 1.4%.

Write to Paul Hannon at paul.hannon@wsj.com