Unionized workers at BHP Billiton Ltd.'s (BHP) Spence copper mine in Chile unanimoulsy voted in favor of going on strike next week if government mediation fails, a union leader said late Wednesday.

Of the 560 union members at the mine, 523 cast their votes between Monday and Wednesday and all 523 voted for the strike, union spokesman Francisco Aravena said.

Following local labor laws, either the union or mine management can now seek a five-day, government-supervised mediation period.

"The company already told us they'd be seeking the mediation," Aravena said, adding the strike could only begin Oct. 8 after mediation has failed.

Local laws allow for a second five-day mediation period but only if both parties request it. BHP representatives weren't immediately available for comment.

-By Carolina Pica, Dow Jones Newswires; 56-2-820-4244; carolina.pica@dowjones.com