May 31 (Reuters) - Prices of antimony, a strategic metal used in flame-retardants, batteries and munitions, are rising to record highs as solar sector demand outstrips supply, causing a wide deficit with little sign of easing, smelters and analysts say.

The surge in prices, which industry participants expect to persist, underscores the West's vulnerability in relying on top producer China for key minerals and could also force end-users to find alternatives for some applications.

Antimony ingot in China climbed to a record 127,500 yuan ($17,588.88) per metric ton on May 29, up 56% in 2024, data from the Shanghai Metals Exchange showed. European prices have also climbed to a record $21,000 a ton, up 75% this year, Fastmarkets data showed.

Globally, declining ore grades and depleting mines are squeezing antimony supply, Chinese investment bank CICC said in a report.

"The surge has been almost entirely supply driven. It is not clear when the supply constraints will improve," said CRU analyst Chetan Soni, citing various supply disruptions in Myanmar, Oman, Tajikistan and Vietnam.

China, one of the world's top antimony producers and users for more than a century, accounted for 48% of global antimony mine production last year, U.S. Geological Survey data showed, although its reserves fell to 640,000 tons, down from 950,000 tons in 2012.

Antimony supplies from Russia, the world's fifth-largest producer, have been disrupted by Western sanctions over Moscow's invasion of Ukraine, producers, traders and analysts said. Russia accounted for 24% of China's antimony supply last year, but Chinese customs data shows there were no shipments in March and April.

A sales manager at Chinese smelter who declined to be identified said that producers of finished atimony who don't have their own ore supplies and must procure from elsewhere are operating at just 25% capacity.

"The problem is there is not sufficient ore", said a sales manager at a second Chinese smelter.

Increasing demand for arms and ammunition due to wars and geopolitical tensions is likely to see tightening control and stockpiling of antimony ore, analysts at China Securities said in a note.

Christopher Ecclestone, principal and mining strategist at Hallgarten & Co, said "clandestine" western military buying is also driving antimony demand. "The supply crisis is not going away and the military have bottomless pockets," he said.

China Merchants Securities forecasts antimony demand from the photovoltaic sector, where the metal is used to improve the performance of solar cells, will increase to 68,000 tons in 2026 from 16,000 tons in 2021, with the sector's share in total consumption rising to 39% from 11%. It expects the supply gap will expand to 21,000 tons by 2026 from 8,000 tons last year.

"It's basically difficult to see a quick ramp up in supply, but the market at the moment probably needs in excess of 10,000 tons of material to cut the deficits," said Nils Backeberg, an analyst at consultancy Project Blue, who expects prices to be at $20,000 per ton over the longer term.

"At the current prices, we will see impacts to the demand market," he said. "There will be substitutions, there will be alternatives being used, but there will be some time in getting those alternatives."

Rising antimony prices have pushed the share prices of Chinese producers including Hunan Gold, Tibet Huayu Mining and Guangxi Huaxi Non-Ferrous up between 66% and 95% in 2024.

More supply takes years to reach fruition, though governments are making efforts to find new sources.

In April, Perpetua Resources Corp received a letter of interest from the U.S. Export-Import Bank for a loan up to $1.8 billion to develop an antimony and gold mine in Idaho, part of Washington's efforts to offset China's critical minerals dominance.

Perpetua's Stibnite mine would be the only U.S. antimony source and according to the company could meet 35% of U.S. demand in its first six years. The Department of Defense has committed nearly $60 million to fund its permitting process, which has lasted eight years, to boost U.S. production for bullets and other weaponry.

($1 = 7.2489 Chinese yuan)

(Reporting by Amy Lv in Beijing and Seher Dareen in Bengaluru; Additional reporting by Pratima Desai, Polina Devitt in London, Melanie Burton in Melbourne and Ashitha Shivaprasad in Bengaluru and Ernest Scheyder in Houston; Editing by Tony Munroe and Christian Schmollinger)