Britain's Prime Minister Theresa May criticized the U.S. for imposing tariffs | Darren Staples — WPA Pool/Getty Images Theresa May: Trump’s trade tariffs ‘unjustified’ British PM says EU ‘should be permanently exempted from tariffs.’

U.K. Prime Minister Theresa May criticized U.S. President Donald Trump's "unjustified decision" to slap tariffs on EU steel and aluminum.

The tariffs — 25 percent on steel and 10 percent on aluminum — affect the European Union, Mexico and Canada, and came into effect Friday.

In a statement, May said: “I am deeply disappointed at the unjustified decision by the U.S. to apply tariffs to EU steel and aluminium imports.

“The U.S., EU and U.K. are close allies and have always promoted values of open and fair trade across the world.

“Our steel and aluminium industries are hugely important to the U.K., but they also contribute to U.S. industry including in defence projects which bolster U.S. national security.”

She added that the EU and U.K. “should be permanently exempted from tariffs.”

The EU, Mexico and Canada were previously given temporary exemptions from the duties, which Washington said were necessary because of national security concerns. In the case of Canada and Mexico, the U.S. had hoped to address its concerns in the context of ongoing efforts to renegotiate NAFTA, but those talks have taken longer than expected and there is now no precise end date in sight, U.S. Commerce Secretary Wilbur Ross said. Talks with the European Union made some progress, but not enough to warrant a permanent exemption or another temporary exemption, he said.

The EU said it would retaliate against the U.S. move.

“We will now trigger a dispute settlement case at the WTO, since these U.S. measures clearly go against agreed international rules. We will also impose rebalancing measures and take any necessary steps to protect the EU market from trade diversion caused by these U.S. restrictions,” said European Commissioner for Trade Cecilia Malmström on Thursday.