A ruling striking down Trump's use of the 1977 International Emergency Economic Powers Act, or IEEPA, to quickly impose broad global tariffs also would eliminate a favorite cudgel to punish countries that draw his ire on non-trade political matters.

Businesses, lawmakers and former US officials are pressing the US Supreme Court to rule against Donald Trump’s use of emergency tariff powers ahead of a showpiece hearing this week. About 40 legal briefs have been filed by groups ranging from the US Chamber of Commerce to former national security officials in opposition to the signature policy the US president has relied on to wage his trade wars.

President Donald Trump said he would skip attending the Supreme Court hearing this week over the legality of his worldwide tariffs regime.

Canadian Prime Minister Mark Carney said he told Ontario’s premier not to run an anti-tariff ad that angered U.S. President Donald Trump

The Supreme Court will decide whether Trump's tariffs under the IEEPA were legal, a ruling that could expand or limit presidential trade authority.

President Donald Trump has said he feels an “obligation” to watch in person as the US Supreme Court weighs his powers to impose tariffs on much of the world. If Trump does, he will make history as the first sitting president ever to attend oral arguments at the nation’s highest court.

Canadian Prime Minister Mark Carney said he apologised to Donald Trump for an anti-tariff advertisement produced by Ontario that so enraged the US president he cancelled trade talks between the allies.

Trump suspended trade talks with Canada and said he would impose an additional 10% tariff on Canadian imports in response to the advert last week. "I did apologise to the president," Carney told reporters at the Asia-Pacific Economic Co-operation Summit in South Korea on Saturday.