The Commission, which negotiates trade policy on behalf of the 27 EU member states, said Washington must provide "full clarity" on the steps it intends to take following the court ruling.
After the court struck down Trump's global tariffs on Friday, the US president announced temporary, across-the-board tariffs of 10 per cent, which he then hiked to 15 per cent a day later.
"The current situation is not conducive to delivering 'fair, balanced, and mutually beneficial' transatlantic trade and investment, as agreed to by both sides" in the joint statement setting out the terms of last year's trade agreement, the Commission said.
"A deal is a deal."
The comments were far more strongly worded than the Commission's initial response on Friday, which had said only that it was studying the outcome of the Supreme Court decision and keeping in contact with the US administration.
Last year's trade deal set a 15 per cent US tariff rate for most EU goods, apart from those covered by other sectoral tariffs such as on steel. It also allowed zero tariffs on some products such as aircraft and spare parts. The EU agreed to remove import duties on many US goods and withdrew a threat to retaliate with higher levies.
"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," the EU executive said, adding that unpredictable tariffs were disruptive and undermined confidence across global markets.
It said that EU Trade Commissioner Maros Sefcovic had discussed the issue with US Trade Representative Jamieson Greer and Commerce Secretary Howard Lutnick on Saturday.
None of the countries that had reached trade deals with the US had indicated plans to withdraw following the Supreme Court decision, Greer said on Sunday.
"I haven't heard anyone yet come to me and say the deal is off," Greer told CBS News.
"They want to see how this plays out."
