The mines together account for four per cent of the world's copper production.

Union leaders said 95 per cent of the work force at the two mines joined the strike, the first at the Chuquicamata complex since 1996.

Strike action

Humberto Fernandois, Codelco's human resources chief, said the company was  "monitoring the general situation and it is peaceful".

"We did everything possible to avoid it, but it could not be done. This is not good for the country"

Michelle Bachelet,
Chile's president

He said that Codelco stood to lose $8m per day of strike action.

Both mines belong to Codelco, the National Copper Corporation of Chile, which produces around 1.6 million tonnes of copper per year and employs around 5,600 workers.

Some banners posted outside mining headquarters proclaimed: "All of Chile is benefiting from copper except Calama."

Calama is a town near Chuquicamata, the biggest open-pit copper mine in the world.

Prices impacted

Copper prices have rocketed by more than 130 per cent since early 2009, driven by signs of global economic recovery after the steep worldwide downturn.

The stoppage has helped drive international copper prices to a 17-month high, though a senior Codelco official said the state-owned giant has enough stocks to honour deliveries early this year.

The strike also comes a fortnight before a January 17 presidential election run-off election and should it continue the it could weigh on the government candidate's popularity.

Michelle Bachelet, Chile's president, said: "We did everything possible to avoid it, but it could not be done. This is not good for the country."

Chile is the world's largest copper producer, with an estimated output of some 5.3 million tonnes in 2008.