(Adds union president's comments, background details) BHP...

(Adds union president's comments, background details)

BHP Billiton Plc (BHP) BLT.L has begun halting operations at its Escondida copper mine in northern Chile, the world's largest, ahead of a planned strike on Thursday, a union leader told Reuters on Wednesday.

The company has said it plans to stop production at the mine during a strike, which the union has warned could be lengthy, potentially affecting global supplies of a metal used in everything from construction to telecommunications.

"Right now the sulfide leaching plant has stopped, the electrowinning B1 and B2 plants have stopped and the other plants are stopping and what they (the company) are going to do is stockpile (copper)," said Patricio Tapia, the president of Escondida Union No.1.

BHP Billiton said on Tuesday it would halt production during the strike because it could not guarantee the safety of the 80 workers the government had authorized to remain at the mine to perform "critical duties", such as equipment upkeep and adherence to environmental protocols.

"If the company said it's not going to be producing, then they don't need so many workers," Tapia said. The union will bring to court the issue of how many workers will remain at Escondida during the work stoppage, Tapia added.

The strike is scheduled to start at 8 a.m. local time (1100 GMT) on Thursday.

Copper prices rose on Wednesday after the world's top two mines said strikes and permit delays would force them to cut output, squeezing global supply.

Escondida is majority-controlled by BHP, with Rio Tinto (RIO) RIO.L and Japan's JECO 7768.T also holding stakes.