Copper rose the most in more than two weeks as miners prepared to strike in Chile , the world’s biggest producer.

Copper-mine workers in Chile will hold their first nationwide strike tomorrow to push for greater job security , with unions vowing to halt all mining activity. Employees from mines operated by Anglo American Plc (AAL) , BHP Billiton Ltd. (BHP) and state-owned Codelco will put down tools for 24 hours, Raimundo Espinoza, who heads Codelco’s union and represents workers on the company’s board, told reporters in Santiago today.

“The firmer tone was due in part to news that Chilean unions representing Codelco will announce a planned nationwide strike date,” Edward Meir , an analyst at INTL FCStone in New York , said in a report. “Copper production over the past year has been rather trouble-free.”

Copper futures for delivery in May added 0.8 percent to settle at $3.371 a pound at 1:15 p.m. the Comex in New York, the biggest gain since March 22. The metal has posted three straight weekly losses.

The union announcement comes after a three-week strike by Chilean port workers that restricted copper shipments.

Money managers increased their net-short position , or wagers on falling copper prices, to a record 38,951 Comex futures and options contracts as of April 2, according to the U.S. Commodity Futures Trading Commission. That’s the most bearish for government data that goes back to 2006.

Some investors bought to close out bets on lower prices, Andrew Silver , a broker at Triland Metals Ltd. in London, said by e-mail today.

The number of London Metal Exchange copper futures outstanding rose 3.2 percent in the week ended April 4 as prices fell 1.3 percent, indicating new short positions. The fee to borrow copper for a day on the LME jumped today to $3 a metric ton, the highest this year.

On the LME, copper for delivery in three months climbed 0.6 percent to $7,450 a ton ($3.38 a pound).

Tin, lead, aluminum, nickel and zinc gained in London.

Current copper price is $3.46.