Donald Trump announced on Saturday that goods imported from both the European Union and Mexico will face a 30% US tariff rate starting 1 August, in letters posted on his social media platform, Truth Social.

The tariff assault on the EU came as a shock to European capitals as the European Commission and the US trade representative Jamieson Greer had spent months hammering out a deal they believed was acceptable to both sides.

The agreement in principle put on Trump’s table last Wednesday involved a 10% tariff, five times the pre-Trump tariff, which the bloc already described as “pain”.

EU trade ministers will meet on Monday for a pre-arranged summit and will be under pressure from some countries to show a tough reaction by implementing €21bn ($24.6bn) in retaliatory measures, which they had paused until midnight the same day.

In his letter to Mexico’s leader, Trump acknowledged that the country had been helpful in stemming the flow of undocumented immigrants and fentanyl into the United States.

But, he said, the country had not done enough to stop North America from turning into a “Narco-Trafficking Playground”.

“We have had years to discuss our Trading Relationship with The European Union, and we have concluded we must move away from these long-term, large, and persistent, Trade Deficits, engendered by your Tariff, and Non-Tariff, Policies, and Trade Barriers,” Trump wrote in the letter to the EU. “Our relationship has been, unfortunately, far from Reciprocal.”

Claudia Sheinbaum said on Saturday she is sure an agreement can be reached before Trump’s threatened tariffs take effect on 1 August.

Speaking during an event in the Mexican state of Sonora, the Mexican president added that Mexico’s sovereignty is never negotiable.

The higher-than-expected rate has dealt a blow to the EU’s hopes of de-escalation and a trade deal and could risk a trade war with goods of low margins including Belgian chocolate, Irish butter and Italian olive oil.

The EU was informed of the tariff hike before Trump’s declaration on social media.

In a letter to the EU, Trump warned that the EU would pay a price if they retaliated: “If for any reason you decide to raise your Tariffs and retaliate, then, whatever the number you choose to raise them by, will be added onto the 30% that we charge.”

The European Commission president, Ursula von der Leyen, said the 30% rate would “disrupt transatlantic supply chains, to the detriment of businesses, consumers and patients on both sides of the Atlantic”.

She said the bloc was one of the more open trading places in the world, and still hoped to persuade Trump to climb down.

“We remain ready to continue working towards an agreement by August 1. At the same time, we will take all necessary steps to safeguard EU interests, including the adoption of proportionate countermeasures if required,” she said.

The French president, Emmanuel Macron, called on the bloc to “resolutely defend European interests”. Expressing Paris’s “very strong disapproval” of Trump’s announcement, Macron urged the EU to “step up the preparation of credible countermeasures by mobilising all instruments at its disposal” if the two sides failed to reach agreement by 1 August.

Germany’s economy minister, Katherina Reiche, called on the EU to “negotiate in a pragmatic manner”, while the Federation of German Industries (BDI) warned that a trade conflict between the two partners “harms economic recovery, innovation strength, and ultimately confidence in international cooperation.”

Italy’s prime minister, Giorgia Meloni, called for “goodwill  … to reach a fair agreement that can strengthen the west as a whole. It would make no sense to trigger a trade war between the two sides of the Atlantic.” She added that both sides should avoid “polarisation”.

The decision to hike the tariffs will also be another test of Trump’s ability to act in good faith in negotiations.

Brussels will view the latest threat as a maneuver by Trump to extract more concessions from the EU, which he once described as “nastier” than China when it came to trade.

Bernd Lange, head of the European Parliament’s trade committee, said on Saturday that Brussels should react immediately with countermeasures against Trump’s “outrageous” threat to hike tariffs on imports from the European Union.

The EU had been negotiating intensively with Washington for more than three weeks and had made concessions, said Lange.

“It is brazen and disrespectful to increase the tariffs on European goods announced on April 2 from 20% to 30%,” Lange told Reuters.

“This is a slap in the face for the negotiations. This is no way to deal with a key trading partner.”

While Trump indicated earlier this week that his new rates, also levelled against big economies including Japan, South Korea and Brazil, will not apply until 1 August, his latest tactic will create much distrust.

Europe should make it clear that these “unfair trade practices” were unacceptable, Lange said. (Guardian)