The US will not back out of tariff deals it has already sealed with countries around the world, including the UK, the EU, Japan, Switzerland and others, Donald Trump’s trade representative Jamieson Greer said on Sunday.

The US supreme court ruled on Friday that many of the tariffs imposed by the US president were illegal, leading Trump to announce a new 15% global tariff on all imports the next day. But Greer told CBS’s Face the Nation that the new levy was separate from agreements struck in the last nine months with about 20 countries around the world.

“We want them to understand these deals are going to be good deals,” Greer said. “We’re going to stand by them. We expect our partners to stand by them.”

The UK education secretary, Bridget Phillipson, admitted on Sunday that UK businesses faced “uncertainty” after the latest developments, but told Sky News that the UK expected its “preferential” trade arrangements with the US to continue.

Business leaders also said they expected the UK to “double down” on the existing deal announced by Donald Trump and the UK prime minister, Keir Starmer, in May last year, rather than walk away.

The EU said it “stands by its commitments” in the deal struck between Ursula von der Leyen, the European Commission chief, and Trump in Scotland last July. The agreement was for a 15% import tariff on most EU goods – half the rate threatened by Trump at the time.

The bloc is concerned that the deal would be overturned by Trump’s latest 15% levy, which, if imposed on top of existing tariffs, could lead to an even higher rate on many goods including wine and spirits.

In a strongly worded statement, the EU called on the US not to walk back the July deal.

“A deal is a deal,” it said. “As the United States’ largest trading partner, the EU expects the US to honour its commitments.”

It added that EU trade commissioner Maroš Šefčovič had spoken to Greer and Howard Lutnick, the US commerce secretary, on Saturday.

The supreme court ruling on Friday infuriated Trump, who quickly announced the new 15% tariff under the 1974 Trade Act, a different legal framework to the one investigated by judges.

This led to fears in the UK of a hike on the existing 10% tariff on British goods entering the US.

Phillipson described the situation as “evolving” and added: “Of course, we want to get the best possible deal for British businesses.”

She said: “I understand the uncertainty it does cause for them, but they can be assured that we will always be working to make sure they get everything that they need.”

The German MEP Bernd Lange, the chair of the international trade committee at the European parliament, suggested a planned vote next week on the US deal agreed last summer would now be postponed.

The deal has been implemented in the US but not yet in the EU, where it needs parliamentary approval.

Writing on X on Sunday, Lange said: “Pure tariff chaos from the US administration. No one can make sense of it anymore – only open questions and growing uncertainty for the EU and other US trading partners.”

William Bain, the head of trade policy at the British Chambers of Commerce, said he expected the UK to use the latest twist to secure a better long-term deal.

He said the Economic Prosperity Deal (EPD) struck bewteen the UK and the US in May was “never really about the 10% tariff but the carve-outs”, such as retention of the existing 0% tariff on the UK pharma industry, a promise of a reduction in the 25% steel tariffs that has yet to materialise, and a 10% tariff on cars – lower than for some other countries.

“What the EPD deal did was create a framework for negotiations on other sectors such as auto and steel and aluminium,” Bain said. “The government sees value in keeping the deal, and this will give them the perfect opportunity to try to nail the EPD down with legal text.

“My sense is their strategy will be double down on the EPD.”

David Henig, the director of the UK Trade Policy Project at the European Centre for International Political Economy, said: “The nature of dealing with this US administration is no deal can be expected to last, so all governments will be working out what to do next, or what the deal actually is.”

Under the 1974 Trade Act, the new 15% tariff can be applied for only 150 days, meaning Trump will need Congressional approval to extend it beyond 23 August.

Bain asked: “What if Congress doesn’t approve? Could Trump turn around and say: ‘I’m now going to impose 14% tariffs for the next 150 days’?

“What is different now is that Congress is a player, which it wasn’t before, so a whole new route for lobbying by business opens up.”