SANTIAGO – The union at BHP's Escondida mine in Chile, the world's largest copper mine, is expected to overwhelmingly reject the final contract offer from the Anglo-Australian miner, increasing the likelihood of a strike, a union leader told Reuters on Monday.

Union members have until Wednesday to finish voting on the company's proposal, when the union will conduct an official count. After that, either party can call for a period of government-mediated arbitration that could last as many as 10 days.

"Half our members have voted," said union spokesman Carlos Allendes. "We hope for positive and overwhelming results, with around 80% rejecting the offer from Escondida."

BHP declined to comment on the union's latest statements.

Copper prices fell on Monday as investors shrugged off the potential strike at Escondida and focused instead on economic data this week expected to show slowing growth in top metals consumer China.

A 44-day strike at the mine last year jolted global copper markets and slowed economic growth in the South American country, which is the world's top copper producer.

After several meetings last week, unionized workers at Escondida concluded that BHP's final contract offer did not meet union demands, according to an internal document viewed by Reuters.

The company's final offer, presented to workers last week, had included an approximately $18,000 signing bonus, and a 1.5 percent boost to salaries, with increases for inflation, according to a BHP summary of the contract offer viewed by Reuters.

The union in June asked for a signing bonus around twice that now offered by the company, and a salary increase of 5 percent.

The total contract offer, which is equivalent to roughly 18-million pesos ($27,700), also includes a one-time payment as compensation for the end of a housing benefits plan.

Earlier on Monday, workers at Codelco's Chuquicamata copper mine in Chile, the state miner's second largest by output, walked off the job in protest at the "unjustified layoff" of two workers.