Experts say the decision by the US Supreme Court to rule that US president Donald Trump’s tariff system was unlawful still leaves a great deal of uncertainty for global trade.

A day after the court ruling Trump announced that the US will raise global tariffs to 15%.

This is an increase from the 10% rate announced in the hours after the decision after the administration invoked Section 122 a law which has never been put into practice before. The law, which falls under the 1974 Trade Act, gives Trump the power to put in place tariffs up to a maximum of 15% for 150 days, at which point Congress must step in.

The news brought a strong reaction from the European Union which issued a statement in which it said the tariff breaches its recent agreement with the US.

“The European Commission requests full clarity on the steps the United States intends to take following the recent Supreme Court ruling on the International Emergency Economic Powers Act (IEEPA),” It stated.  The current situation is not conducive to delivering ‘fair, balanced, and mutually beneficial’ transatlantic trade and investment, as agreed to by both sides and spelled out in the EU-U.S. Joint Statement of August 2025.

“The Commission will always ensure that the interests of the European Union are fully protected. EU companies and exporters must have fair treatment, predictability, and legal certainty.

“A deal is a deal. As the United States’ largest trading partner, the EU expects the US to honour its commitments set out in the Joint Statement – just as the EU stands by its commitments.

“In particular, EU products must continue to benefit from the most competitive treatment, with no increases in tariffs beyond the clear and all-inclusive ceiling previously agreed. Tariffs are taxes, driving up costs for both consumers and businesses, as recent studies clearly confirm.

“When applied unpredictably, tariffs are inherently disruptive, undermining confidence and stability across global markets and creating further uncertainty across international supply chains.”

Banking giant ING added while there remained uncertainty the view was tariffs will be here to stay.

“While the new universal tariff rate of 15% should theoretically mean a slightly lower average tariff rate, it may be that a simplified tariff structure leads to a higher realised rate,” It explained. “Reduced complexity leaves less room for ambiguity and fraud and lowers incentives to reroute trade through third countries. Remember too, that a significant number of exemptions to the tariffs also apply, such as high-tech products and critical minerals. Consequently, for now, we assume that realised tariff revenue rates remain a little above 10%.”

The bank’s statement continued that while the ruling left open the possibility of refunds on roughly $130bn in tariffs collected under the International Emergency Economic Powers Act (IEEPA), it did not offer guidance on how that would be achieved. The Administration has warned that it would fight, running the risk of years of litigation.

It concluded: “Friday’s Supreme Court ruling sent a strong signal about the limits of presidential power. It can be seen as evidence that checks and balances in the US still work. However, we don’t think that president Trump will use the ruling as a backdoor option to back down on his tariff agenda. On the contrary, announcements since the Supreme Court’s ruling strongly confirm that Trump has no intention of removing his “most beautiful word” from the English dictionary. Uncertainty is back  and given the latest muscle-flexing by European leaders, the risk of escalation is now higher than it was a year ago.”

Matt Britzman, senior equity analyst, Hargreaves Lansdown said despite the political fallout there may be some upside for investors.

“The UK had seemingly done its homework, securing a 10% trade deal with the US, but the White House’s new 15% blanket tariff rather takes the shine off that achievement, dragging the UK back into fresh trade uncertainty,” he explained. “The new tariff uses a different legal authority, but it is a far blunter instrument than the flexible, targeted tools the administration had been relying on. While the White House insists that all existing trade deals remain intact, the EU is already making noises about pausing negotiations until the new landscape becomes clearer.”

Britzman  continued: “Wall Street ended last week on a high after the Supreme Court struck down the Trump administration’s use of its preferred tariff powers, with investors quickly repricing the outlook for lower effective tariff rates. That optimism is fading somewhat this morning, however, with US futures pointing lower as the dust settles and investors strap back in for another wave of tariff uncertainty.

“Taking a step back, though, there’s an argument that this is a positive development for investors. The Supreme Court ruling was largely expected and sends an unambiguous signal that there are limits to the executive’s power over trade and tariffs. Tariff headlines will no doubt continue to dominate the news cycle, but the goalposts have shifted – the near-unlimited use of tariffs under the old powers has come to an end, and for markets that have spent months toying with worst-case scenarios, that’s a small silver lining.”