President Donald Trump signed an order Tuesday doubling U.S. tariffs on steel and aluminum imports to 50%, citing the need to bolster domestic manufacturing and defend national security interests, Bloomberg reported. The new rates took effect at 12:01 a.m. Wednesday in Washington.

The administration justified the hike by saying previous tariff levels had failed to ensure sufficient production capacity in U.S. steel and aluminum industries. A proclamation issued by the White House stated the new duties are aimed at eliminating “the national security threat posed by imports” of these materials and their derivatives.

“Increasing the previously imposed tariffs will provide greater support to these industries,” the official order said.

Bloomberg notes that the policy shift adds friction to ongoing trade negotiations, particularly with a July 9 deadline looming for talks with key U.S. partners. Trump’s decision comes even as a federal court recently struck down many of his earlier tariffs imposed under emergency authority, a ruling that did not affect his current metals duties.

The United Kingdom will remain exempt from the new 50% rate, with its imports continuing to be taxed at the existing 25% levy, according to Bloomberg. This exemption gives both nations room to negotiate new terms before the July cutoff.

Mexico has also responded, with Economy Minister Marcelo Ebrard labeling the tariff increase “unsustainable” and vowing to seek an exemption for his country.

Trump unveiled the tariff hike during a speech last Friday at a U.S. Steel Corp. facility in Pennsylvania, where he also endorsed the company’s sale to Japan’s Nippon Steel Corp. while pledging that U.S. oversight would remain, Bloomberg reported. “At 25%, they can sort of get over that fence; at 50% they can no longer get over the fence,” Trump told steelworkers.

The aluminum tariff was later raised to match steel’s 50% rate via a post on Trump’s social media, according to Bloomberg.

For more details, read the original Bloomberg story: Trump Signs Order Doubling US Steel, Aluminum Tariffs to 50%.