Washington DC - The US Supreme Court on Friday dealt a huge blow to President Donald Trump by ruling that he exceeded his authority in imposing a swath of tariffs that upended global trade.

The conservative-majority high court ruled six-three in the judgment, saying the International Emergency Economic Powers Act (IEEPA) "does not authorize the President to impose tariffs."

While Trump has long used tariffs as a lever for pressure and negotiations, he made unprecedented use of emergency economic powers upon returning to the presidency last year to slap new duties on virtually all US trading partners.

These included "reciprocal" tariffs over trade practices that he deemed unfair, alongside separate sets of duties targeting major partners Mexico, Canada, and China over alleged drug trafficking and immigration.

The court on Friday noted that "had Congress intended to convey the distinct and extraordinary power to impose tariffs" with IEEPA, "it would have done so expressly, as it consistently has in other tariff statutes."

The ruling does not impact sector-specific duties that Trump has separately imposed on imports of steel, aluminum, and various other goods. Formal probes which could ultimately lead to more such sectoral tariffs remain in the works.

The Supreme Court's decision affirms earlier findings by lower courts that tariffs Trump imposed under IEEPA were illegal.

A lower trade court had ruled in May that Trump overstepped his authority with across-the-board levies and blocked most of them from taking effect, but that outcome had been put on hold as the government sought an appeal.