July 18 (Bloomberg) -- Cotton futures fell this week to extend a slump to the longest in 55 years, signaling lower prices for T-shirts and jeans.

On ICE Futures U.S. in New York, cotton for December delivery dropped for the 11th straight week, the longest slump since July 1959, when Bloomberg data starts. The most-active contract settled at 67.74 cents a pound, close to the lowest in more than two years. This year, the fiber has tumbled 20 percent, the most among 22 raw materials in the Bloomberg Commodity Spot Index.

The U.S. Department of Agriculture last week said global inventories next year will increase 5.1 percent from 2014, boosting its forecast from June. The agency increased its estimate for stockpiles in China, the worldΆs top user, by 2.5 percent. Last month, the World Bank cuts its 2014 outlook for global economic expansion.

“There is such a surplus in cotton globally, and the world economy is not growing enough to buy it,” Michael K. Smith, the president of T&K Futures & Options Inc. in Port St. Lucie, Fla., said in a telephone interview. “The demand would have to come from China, and as far as we can tell, their warehouses are basically full, so they are set for a while.”

Output in the U.S., the top exporter, may rise 32 percent this year, according to Plexus Cotton Ltd. In Texas, the biggest state producer, rain in the second quarter sparked a planting surge that will bolster inventories.

“If there is no robust increase in demand, then the price could continue to drop for the next couple months” by as much as 20 cents, Smith said. “I donΆt know where that demand would come from, other than China.”