The European Union announced Friday that it will file a complaint with the Word Trade Organization in response to President Donald Trump’s decision to move forward with tariffs on foreign steel and aluminum.

Calling the protectionist measures “illegal” under international trade rules, EU trade commissioner Cecilia Malmström said the bloc would seek relief from the WTO, the Geneva-based arbitrator of trade disputes.

“We will now trigger a dispute settlement case at the WTO, since the U.S. measures on steel and aluminum clearly go against agreed international rules,” she said at a news conference, according to NPR.

Trump stunned U.S. allies on Thursday by imposing a 25 percent import tax on foreign steel and a 10 percent duty on aluminum. The tariffs were technically implemented in March, but Washington had given exemptions to the EU, Canada, Mexico and other key trading partners pending the outcome of broader trade negotiations.

The move outraged foreign leaders, who denounced Trump’s invocation of national security to justify the across-the-board metal tariffs. Canadian Prime Minister Justin Trudeau said it was “inconceivable” that Canada could be considered a national security threat to the U.S., and French President Emmanuel Macron described the tariffs as a form of economic “war” against America’s allies. (RELATED: Justin Trudeau Says Canada Will Retaliate Against US Over Trump’s Steel Tariffs)

The EU’s complaint with the WTO touches off a dispute settlement process in which trade arbitrators attempt to negotiate a mutual agreement between Washington and Brussels or, failing that, impose “countermeasures” on the losing party. In the meantime, Brussels is preparing “rebalancing measures” that could include tariffs on U.S. bourbon, motorcycles and blue jeans exports, Malmström said.

Also Friday, the EU filed a separate WTO complaint against China over intellectual property violations. Like Washington, Brussels has accused the Chinese government of rampant technology theft, among other complaints about Beijing’s trade policies.

“If players in the world don’t stick to the rule book, the system might collapse,” Malmström said, according to Agence France Presse. “That is why we are challenging the US and China at the WTO.”

Elsewhere, U.S. trading partners were gearing up to levy their own import duties on American goods in retaliation for Trump’s metal tariffs. Canada said it would impose “dollar-for-dollar” tariffs on American steel and aluminum beginning July 1, while Mexico announced plans to tax U.S. exports of flat steel, pork bellies, and several other types of agricultural products.

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