WASHINGTON — President Trump warned on Saturday that he would apply higher taxes on imported European cars if the European Union carried through on its threat to retaliate against his proposed stiff new tariffs on steel and aluminum.

“If the E.U. wants to further increase their already massive tariffs and barriers on U.S. companies doing business there, we will simply apply a Tax on their Cars which freely pour into the U.S.,” Mr. Trump wrote on Twitter from Florida, where he was spending part of the weekend. “They make it impossible for our cars (and more) to sell there. Big trade imbalance!”

It was the latest indication that Mr. Trump, despite pressure from foreign allies and American business leaders, is standing by his decision to impose a 25 percent tariff on steel imports and a 10 percent tariff on aluminum imports from all countries. The action is likely to be signed this coming week.

After Mr. Trump announced the penalties on Thursday, European Union leaders warned that American goods like Kentucky bourbon and Harley-Davidson motorcycles, many of them with roots in the home states of key Republican leaders, would be treated “the same way” if the steel and aluminum tariffs were enforced. Officials from Australia, Canada and other American trade partners also made retaliatory threats.