"If we lose, it will be the end of the United States."

The US Supreme Court "could certainly rule" in favor of the customs duties imposed by Donald Trump, deemed illegal on appeal , one of the US president's advisers said on Sunday.

On Friday, a US federal appeals court ruled that the president lacked the authority to impose many of the tariffs on imported goods he has implemented since returning to power in January. However, these tariffs will remain in effect while the Supreme Court makes a final decision on the case.

On Fox News Sunday, White House trade adviser Peter Navarro claimed the decision, reached by a seven-to-four majority, was politically motivated. He called the justices "politicians in black robes."

Navarro also said the dissenting arguments made by some of the justices were "very, very strong." "I think this provides a very clear roadmap for how the Supreme Court can certainly rule in our favor," he said.

The ruling states, among other things, that the President of the United States cannot impose tariffs on virtually all imports without a time limit. "We never said they would be permanent," said Peter Navarro. "We are very optimistic," he continued, adding that "if we lose this case (...) it will be the end of the United States."

The court had been seized by Democratic states and a coalition of small businesses, which Peter Navarro accused of wanting to continue "buying cheap Chinese crap."