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The US has slapped tariffs on imports of one-kilo gold bars, in a move that threatens to upend the global bullion market and deal a fresh blow to Switzerland, the world’s largest refining hub.

The Customs Border Protection agency said one-kilo and 100 ounce gold bars should be classified under a customs code subject to levies, according to a so-called ruling letter dated July 31, which was seen by the Financial Times. Ruling letters are used by the US to clarify its trade policy.

The CBP’s decision stands in sharp contrast with the industry’s previous expectations that these types of gold bars should be classified using a different customs code that is exempt from Trump’s countrywide tariffs.

One-kilo bars are the most common form traded on Comex, the world’s largest gold futures market, and comprise the bulk of Switzerland’s bullion exports to the US.

Relations between Washington and Bern have deteriorated after the US last week announced a 39 per cent tariff on imports from the country. Gold is one of Switzerland’s biggest exports to the US, customs data shows.

The tariff ruling dealt “another blow” to the Swiss gold trade with the US, said Christoph Wild, president of the Swiss Association of Manufacturers and Traders of Precious Metals. Wild added that the gold tariff would make it difficult to meet its demand for the yellow metal.

Earlier this year traders rushed to bring gold into the US ahead of Trump’s “liberation day” tariffs — building up a record stockpile on Comex, and leading to a temporary shortage of gold in London.

However, when those tariffs were announced, they included exemptions for many commodities including a certain classification of bullion which was widely interpreted to cover large gold bars.

The global trade flow for bullion is normally triangular: large gold bars travel between London and New York, via Switzerland, where they are recast into different sizes.

The two markets use different-sized bars, with London using a 400 troy ounce bar, which is about the size of a brick, while the kilo bar, roughly the size of a smartphone, is preferred in New York.

Bullion has been on a historic rally this year, rising 27 per cent since the end of 2024 and briefly touching $3500 per troy ounce. Fears over inflation, concerns over government debt levels, and the decline of the US dollar as a reserve currency have all contributed to gold’s surge.

Switzerland exported $61.5bn of gold to the US over the 12 months ending in June. That same volume would now be subject to an additional $24bn in tariffs under Switzerland’s 39 per cent tariff rate, which went into effect on Thursday.

“The prevailing view was that precious metals remelted by Swiss refineries and exported to the US could be shipped tariff-free,” said Wild, the association president. “However the custom code classification for different gold products is not always precise.”

Several Swiss gold refineries said they had spent months with lawyers to determine what types of gold products might be exempt, or not. Two refineries told the FT they had temporarily reduced or halted shipments to the US as a result of the uncertainty.

The ruling letter, which was a response to a formal request for clarification from a Swiss refinery, said one-kilo and 100 ounce bars fell under classification code 7108.13.5500, and stated that they did not fall under code 7108.12.10 — the only code exempt from duties.