Former Federal Reserve chairmen Ben Bernanke and Alan Greenspan and other leading economists are warning President Donald Trump that new tariffs on foreign steel would raise costs for U.S. manufacturers, increase prices for consumers and harm relations with allies. Countries from Canada to China have been bracing for the result of a Commerce Department investigation into whether steel imports threaten U.S. national security. Officials within the administration have been debating several options, including raising tariffs on steel imports, according to people briefed on the discussions. “Additional steel tariffs would actually damage the U.S. economy,” according to a letter published Wednesday by the American Action Forum, a Washington-based think tank. “Tariffs would raise costs for manufacturers, reduce employment in manufacturing, and increase prices for consumers.” The letter is signed by 15 former chairs of the White House council of economic advisers. In addition to Bernanke and Greenspan, they include Nobel laureate Joseph Stiglitz, Glenn Hubbard, Christina Romer and Martin Feldstein. The economists note that the U.S. already has over 150 duties on steel imports, some as high as 266 percent. They point out that America’s top sources of steel include allies such as Canada, Brazil, South Korea and Mexico. “Additional tariffs would likely do harm to our relations with these friendly nations,” they said in the letter. Trump ordered the Commerce Department earlier this year to probe steel imports under the seldom-used Section 232 of the Trade Expansion Act of 1962, which allows the president to impose trade measures if the government finds evidence of a national-security threat from foreign shipments. While Trump and members of his cabinet have accused China of dumping cheap steel on global markets, tariffs may not do much to reduce excess supply, since the U.S. imports a relatively small amount of its steel from Chinese producers.