SANTIAGO, June 18 (Reuters) - Anglo American's copper output at its Los Bronces mine in Chile is expected to fall nearly a third from average historical levels next year as the miner pauses a processing plant for maintenance, the company's Chile head said on Tuesday.

Anglo American previously said it would suspend the plant, which is the smaller and older of two at the site, in mid-2024, but had not disclosed the estimated hit on production.

Los Bronces is Anglo American's flagship copper mine, representing about a fourth of its copper output worldwide, which spans several sites in Chile and Peru.

Demand for the red-metal has surged worldwide, and BHP made an unsuccessful $49 billion offer for rival Anglo American this year in a play for its smaller rival's copper mines.

The maintenance on the processing plant is expected to take at least a couple of years, said Patricio Hidalgo, CEO of Anglo American in Chile, at an event.

"It will depend on the input and the ore grade, so it's not one-to-one, but it's about 30% as of next year, compared to what we historically have produced," he told reporters.

Anglo American is aiming to compensate for falling production levels dinged by hard, lower grade ore, and hopes to start new extraction in 2027, after delays due to permitting and other factors.

Suspending the processing plant, called Los Bronces, will give the company time to move ahead with preparations for new extraction phases, including securing an additional water supply and moving the Perez Caldera tailings dam.

"It opens a window for us," Hidalgo said. "We have to see when the optimal window of time is to remove the dam."

Los Bronces is majority owned by the miner's local unit, Anglo American Sur, with a stake just over 50%. A joint venture between Japan's Mitsui and Chile's state-run Codelco owns nearly 30%, while Japan's Mitsubishi owns the remainder. (Reporting by Fabian Cambero; Additional reporting by Daina Beth Solomon; Editing by Anthony Esposito and Sandra Maler)