UPDATE 1-Uniqlo sales mark first fall in 6 months in January

* Company attributes January fall to shortage of winter items

* "Heattech" inventory nearly sold out in December

TOKYO Feb 2 (Reuters) - Japan's Fast Retailing (9983.T) said sales at its Uniqlo casual-clothing chain fell 7.2 percent in January, the first year-on-year decline in six months, blaming a shortage of inventory after robust sales the previous month.

Uniqlo has defied a prolonged retail slump in Japan as thrifty consumers snap up its relatively inexpensive merchandise, highlighted by stellar same-store sales growth of more than 30 percent in September and October.

But the company said sales fell last month due to a shortage of popular winter items such as its "Heattech" line of basic garments, such as T-shirts and leggings, made of heat-trapping fabric.

Anticipating strong demand, Uniqlo ordered 80 percent more Heattech garments for the 2009/10 winter than the previous winter, but it nearly sold out of this supply in December, it said.

Before the announcement, shares of Fast Retailing ended flat at 15,030 yen, underperforming a 1.6 percent gain in the benchmark Nikkei average .N225. (Reporting by Taiga Uranaka; Editing by Chris Gallagher)