Copper futures fell to the lowest in two months after stockpiles surged and Barclays Plc flagged concern over growth in China, the world’s biggest consumer.

Inventories tracked by exchanges in Shanghai, London and New York climbed five straight days last week to the highest since May. Barclays said in a report that a recent slump in the copper market “may be an early warning sign of weakening in China’s economic momentum.” Futures have fallen for four of the past five weeks on the Comex.

“Copper prices are super weak and look to continue to weaken,” Phil Streible, a senior market strategist at RJO Futures in Chicago, said in a telephone interview. “Prices are going to erode lower and probably see shorts add to their positions here.”

Copper futures for December delivery fell 0.1 percent to $2.0815 a pound at 9:35 a.m. on the Comex in New York, after touching $2.076, the lowest since June 24.

The metal slid as much as 0.6 percent on the Shanghai Futures Exchange before closing little changed at 36,420 yuan ($5,455) a metric ton. It dropped 2.6 percent last week, the most since May 13.

Hedge funds and other large speculators held a net-short position of 4,991 U.S. copper futures and options contracts in the week ended Aug. 23, according to Commodity Futures Trading Commission data released three days later. They switched from a net-long position of 2,237 a week earlier.

Copper has wiped out all of its gains this year, lagging behind other metals, as supply continues to outstrip demand. Holdings in warehouses tracked by the London Metal Exchange climbed every day last week to the most since 2015 on Friday, with stockpiles in Asia surging to the most since 2013. The metal is likely being exported from China to the LME’s Asian warehouses to take advantage of more favorable premiums, Barclays said in its report, dated Aug. 28.

Chinese growth is forecast to slow to 6.5 percent this year, the weakest since 1990. Copper will be in “substantial surplus” for at least the next two years as a “huge wave of new mine supply hits the market,” according to Barclays.

London markets were closed Monday for a holiday.