Plaintiffs argue president lacks legal power to impose duties under the International Emergency Economic Powers Act

Trump has underpinned his tariff policy with an emergency power that is now being challenged as unlawful in the federal courts. On Thursday the US court of appeals for the federal circuit will hear oral arguments in the case, VOS Selections v Trump.

A group of small business owners are suing the US president on grounds that he lacks legal authority from Congress to impose severe tariffs that could damage their bottom line. The Trump administration has invoked a 1977 law, the International Emergency Economic Powers Act (IEEPA), claiming that various national emergencies – including US trade deficits with trading partners and the scourge of fentanyl trafficking – demand urgent action.

But the plaintiffs have countered that the IEEPA does not give the president the power to impose tariffs, and has never been used in such a way in its almost half a century on the statute books.