SANTIAGO The union of supervisors at Chile's Escondida mine is gearing up to strike at the world's largest copper deposit and is almost certain to reject a contract offer from mine owner BHP, the union's president said on Friday.

Though the sprawling Escondida mine could continue to operate even if supervisors walked off the job, a strike could lead to production bottlenecks or slowdowns.

"I have no doubt that the strike will win," union head Alexis Barrera said in an interview, adding that nearly all members have already cast their votes.

Voting on a proposal from BHP, a multinational miner, closes on Friday afternoon. There was no immediate comment from BHP.

The government could launch a five-day mediation process that can be extended another five days if the parties come close to reaching a deal.

Mediation will happen "as long as the company asks for it," Barrera said. If BHP does not request mediation proceedings, the strike would begin on Oct. 4.

The union accuses BHP of looking to reduce benefits and production bonuses while extending working hours.

Chile's powerful Union 1 workers' union said it would stand in solidarity with the Escondida supervisors and ensure that no "illegal replacements" came on the job in the event of a work stoppage.

Escondida produced 1.05 million metric tons of copper in 2022.

(Reporting by Fabian Cambero; Writing by Kylie Madry; editing by Rami Ayyub)