U.S. President Donald Trump has doubled tariffs on steel and aluminum imports from 25% to 50%, citing the difficulty of reaching an agreement with Chinese President Xi Jinping. The move comes just before trade negotiations are set to take place on Wednesday in Paris with several of Washington’s key trading partners.

The executive order announcing the tariff increase was published on Tuesday and went into effect at midnight Washington time. President Trump justified the decision by saying, “We want to ensure that these imports do not threaten national security.”

The text of the order stated: “Although the existing tariffs have provided essential price support in the U.S. market, they have not enabled these industries to raise and maintain their capacity utilization rates at levels sufficient to ensure their viability or to meet national defense requirements.”

The United Kingdom was exempted from the new increase, with tariffs on its exports remaining at 25%. The exemption aims to allow time for London and Washington to finalize a trade agreement announced last month, which is expected to eventually exempt British steel from tariffs.