The US Supreme Court justices have raised doubts over the legality of President Donald Trump’s use of the International Emergency Economic Powers Act of 1977 to unilaterally set tariffs on imports from more than 100 countries in an attempt to reduce the trade deficit and ignite more manufacturing in the United States.

The case, seen as one of the most significant economic cases in years, will determine whether President Trump acted within his powers or overstepped Congress’s authority to levy taxes. Chief Justice John Roberts and conservative justices Amy Coney Barrett and Neil Gorsuch expressed scepticism over the government’s justification for the tariffs.

Several small businesses and a group of states have challenged the duties as unlawful. Trump’s attorney faced tough questioning, including on the use of federal law to impose reciprocal tariffs on all countries. The lawyer arguing on behalf of a group of private companies told the justices that common sense makes clear that the administration’s interpretation of IEEPA is flawed. Some of the conservative justices also signalled that they were wrestling with their recognition of the inherent power that presidents have in dealing with foreign countries.