By Jon Sindreu

LONDON--Retail sales in the U.K. fell less than expected in September and registered the 18th consecutive month of annual growth.

The quantity bought in the retail industry increased by 2.7% compared with September 2013, the Office for National Statistics said on Thursday. Month-on-month, however, volumes were 0.3% lower than in August.

While still pointing to a slight slowdown in the retail sector, these figures are stronger than economists' forecasts and suggest consumer spending--which boosted economic growth in Britain during the second quarter--remained strong in the three months to September. As the U.K.'s main trade partner, the eurozone, faces increased economic woe, British recovery is highly dependent on its own consumers.

The biggest contributor to the rise in sales was non-store retailing--which include Internet retailers and street markets--while the steepest fall was in clothing, where sales were the lowest since April 2012. According to the ONS, consumers delayed purchases of autumn and winter clothes due to the weather being warmer than usual.

The overall increase in sales was brought about by lower prices. Average store prices dropped 1.4% on the year, the largest decrease in more than five years, with petrol stations and food stores leading the fall. Stark competition amongst supermarket stores during the year, the ONS said, has caused discounters to gain market share and traditional retailers to slash prices.

This is consistent with headline inflation figures for September released last month, which showed prices in the U.K. growing slower than expected, driven by food and fuel.

Write to Jon Sindreu at jon.sindreu@wsj.com