Unionized workers at BHP Billiton Ltd.'s (BHP) Spence copper mine in Chile rejected the latest contract offer and will likely vote in favor of a strike, a union leader said Friday.

In general assemblies held Thursday and Friday, workers voted to reject the offer.

Following local labor laws, a strike vote, via secret ballot, will be held on Sept. 27, 28 and 30, union spokesman Francisco Aravena told Dow Jones Newswires.

"One hundred percent of workers rejected the offer at the assemblies - we think this is headed for a strike," Aravena said.

He added that the 560-strong union isn't going to seek the five-day, government-assisted mediation period, but is willing to restart talks if the mining company sought the mediation.

"We're gearing up for a strike," Aravena said.

Current contracts expire on Sept. 30. Spence workers are seeking a 5.5% wage increase, social benefits and bonuses linked to international copper prices.

"Collective bargaining is a normal process in our organization's labor relations and we approach it as an opportunity to advance towards the future along with our workers," BHP Billiton said in a statement released earlier this week.

The union represents about 95% of the workers eligible for union membership and about 60% of the mine's total workforce, according to the union's calculations.

The open-pit Spence mine produced about 165,000 metric tons of copper, in the form of cathodes, last year. This year, it's expected to produce about 200,000 tons of copper.

This is Spence's second collective bargaining process as workers negotiated the about-to-expire contract in 2006 before the mine went into production.

In addition to Spence, in Chile BHP owns the Cerro Colorado mine and controls and operates the Escondida copper mine. The latter will begin wage negotiations later this year.

In 2006, when copper prices were booming, Escondida workers went on strike for nearly a month, bringing the world's largest copper mine to a standstill as they sought higher wages and production bonuses.

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