By Emese Bartha

FRANKFURT--German retail sales in September suffered their biggest monthly fall since May 2007, adding to concerns that private consumption won't be as strong a support to growth in Europe's biggest economy as previously hoped.

Retail sales in September fell 3.2% in calendar-and inflation-adjusted terms, compared with August, the Federal Statistics Office said Friday. That was a significantly bigger drop than the 0.8% decline economists had forecast in a Dow Jones Newswires survey. Also, retail sales growth for August was revised down to 1.5% from a first estimate of 2.5%.

Retail sales are very a volatile indicator, prone to large revisions.

On an annual basis, retail sales were up 2.3%, the statistics office Destatis said. In the first nine months of the year retail sales increased 1.3% in real terms compared with a year earlier.

The annual figures showed a 4.2% rise in food, beverages and tobacco sales, and a 1.0% rise in non-food items in real terms. Elsewhere, however, sales in textiles, clothing, shoes and leather goods plunged 7.3%, Destatis said.

The dismal data come as surveys suggest that consumer sentiment in Germany is set to improve. The GfK survey released last week showed that German consumer confidence is expected to rebound in November, indicating that German consumers may have become less concerned about recent global conflicts and their impact on economic growth.

Write to Emese Bartha at emese.bartha@wsj.com