Copper prices dropped Wednesday the most in 19 months after a jump in inventories increased worries about an economic slowdown in China, the world's largest consumer of the metal.

Copper futures for July delivery settled 3.49 percent lower at $2.5435 a pound, below their 50-day moving average for the first time since April 6 and their worst drop since September 2015. Prices of the metal continued falling in extended trade to near $2.53 a pound.

"I think the biggest thing is the worries about China," said Ryan McKay, associate commodities strategist at TD Securities.

"First we had a large build in LME copper stocks today, adding concern to the already elevated inventory levels," McKay said. "Also manufacturing data from China this week has come in below expectations, raising concerns around Chinese demand."

Copper performance so far this week

Source: FactSet

On-warrant inventories available for delivery at LME-registered warehouses increased by 38,950 tonnes, or 32 percent, to 160,200 tonnes, the highest since mid-April, according to Reuters.

Late Monday Eastern Time, the Caixin/Markit manufacturing PMI, a key survey for the Chinese economy, for April showed a greater-than-expected decline to 50.3, the lowest since September 2016.

Copper is a key material used in home construction, industrial manufacturing and consumer goods around the world. The Chinese government injected large amounts of loans last year in an attempt to prevent a sharp slowdown in the world's second largest economy. While recent economic reports have improved, the stimulus has also slowed.

For copper, "demand conditions are still good, just not as good they were three to six months ago," said Dane Davis, commodities research analyst at Barclays. "The stimulus is wearing off and it's starting to feed through in real prices, particularly copper and iron ore."

Iron ore futures fell more than 6.5 percent Wednesday.

Earlier in the week, copper prices leaped to their highest in nearly a month. Traders expected prices to rise given a planned month-long strike at Freeport-McMoran's Grasberg mine in Indonesia.

— CNBC's Gina Francolla and Reuters contributed to this report.