A US federal appeals court has ruled that most of President Donald Trump's tariffs are illegal, opening the door to legal challenges to his much-discussed foreign and trade policies, BBC reports.

In a 7-4 decision, the court said Trump used the International Emergency Economic Powers Act (IEEPA) to impose tariffs, but the law does not give the president such authority. The ruling also noted that imposing tariffs is a constitutional prerogative of Congress.

The new ruling will take effect on October 14. Before that, the administration can appeal to the Supreme Court. Analysts say the case is now almost certain to go to the highest court.

President Trump has currently imposed unfair tariffs on most countries in the world, such as China, Mexico, and Canada.

However, in a post on Truth Social, Trump criticized the ruling, saying that if allowed to stand, the decision would literally destroy the United States.

"Today an appeals court said we should lift the tariffs, but they know the US will win in the end. This ruling is not right," he wrote.

He said, "If the tariffs are lifted, it will be a complete disaster for the country. It will make us financially weak. But we have to be strong."

Trump has declared a national emergency over trade. He has argued that the trade imbalance is detrimental to US national security. But the courts have ruled that imposing tariffs is not within the president's authority and that setting tariffs is part of Congress's substantive powers.

Trump claimed to have been authorized to impose tariffs under his emergency economic powers, but the arguments in favor of his claim were rejected by a US federal appeals court, which ruled that his tariffs were unconstitutional and illegal.