By Eric Onstad and Brijesh Patel

LONDON/BENGALURU, April 30 (Reuters) - Copper prices are due to languish in coming months on fear that volatile U.S. tariff policies will curb global growth, including in top metals consumer, China, according to a Reuters poll.

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Benchmark copper prices have eased by 8% since peaking above $10,000 per metric ton in March, sinking to their lowest in over 16 months in April as a trade war heated up between Washington and Beijing.

"A global trade war is bearish for copper and other industrial metals so we expect the downtrend in metals to continue, at least in the short to medium term," said Ewa Manthey at ING.

"And if trade tensions intensify and we see more retaliatory measures, this will add to the bearish sentiment for industrial metals."

The cash copper CMCU0 contract on the London Metal Exchange $(LME.AU)$ should average $9,083 per ton in the third quarter of 2025, a median forecast of 24 analysts showed, down 5.4% from $9,600 in the last poll in January and compared to a closing LME price of $9,430 on Tuesday.

Analysts have more than doubled their consensus forecast of a market surplus in 2025 of the metal often used as a barometer of the global economy to 60,560 tons from 23,000 tons in the previous poll.

ALUMINIUM TO SHIFT TO SURPLUS

Weaker global demand for aluminium and the planned restart of Alcoa's AA.N San Ciprian smelter in Spain is expected to help move the market into oversupply this year.

The surplus of the metal used for transport, construction and packaging, however, will be kept in check due to output in top producer China approaching the government's capacity limit of 45 million tons.

"Barring a global recession, the downside looks limited, but the upside is capped as well, unless China engages in major and broad-based metals-intensive stimulus measures," said Carsten Menke at Julius Baer.

LME cash aluminium CMAL0 is expected to average $2,463 a ton in the third quarter, down 3.4% from the previous poll.

Analysts sharply shifted their market balance forecasts with a swing in their consensus to a surplus of 280,000 tons in 2025 from a deficit of 8,000 tons in the previous poll.

ZINC INVENTORIES EYED

Zinc, mainly used to galvanise steel, has been the worst performing metal so far this year, mainly due to expectations of improved mine supply.

"Inventories remain at a historically low level so an improvement in demand could drive a rally in prices if supply is slow to respond due to recent operating challenges," said Laura Chan at RBC.

Analysts expect LME cash zinc CMZN0 to average $2,675 a ton in the third quarter, down 6.9% from the consensus in the previous poll of $2,874.

They have cut their consensus for a global zinc surplus in 2025 to 12,000 tons from oversupply of 147,000 tons forecast in October, while next year they expect a surplus of 192,000 tons.

(Reporting by Eric Onstad and Brijesh PatelEditing by Tomasz Janowski)

((eric.onstad@thomsonreuters.com; +44 20 7542 7093; Twitter https://twitter.com/reutersEricO; Reuters Messaging: eric.onstad.thomsonreuters.com@reuters.net))