WASHINGTON — Europe’s top trade diplomat is bracing for a trade war with the United States and said on Wednesday that the European Union was preparing to strike back if President Trump followed through with his threat to impose tariffs on Europe’s cars.

The European Union and the United States are meeting this week to try to lay the foundation for trade talks that would reduce tariffs on a range of industrial goods. But as Mr. Trump and members of his administration vent about the pace of negotiations and continue dangling potential auto tariffs, the Europeans have begun drawing up a list of American products they would tax if the trade dispute escalates.

“We have already suffered from the tariffs on steel and aluminum, and we think that they are deeply unjustified,” Cecilia Malmstrom, the European Union trade commissioner, told reporters in Washington on Wednesday. Europe has already challenged the United States at the World Trade Organization over its imposition of 10 percent tariffs on aluminum and 25 percent on steel and has retaliated with tariffs on American goods like peanut butter, whiskey and orange juice.

“We would do the same were we to be targets of tariffs on cars and car parts,” Ms. Malmstrom said.

Ms. Malmstrom said that she had prepared a draft list of American sectors that could face retaliation but that they needed to be approved by member states of the European Union. While she would not detail the products, she said that Europe could retaliate against a range of American goods, including autos and agriculture, and that the response would be fully compatible with World Trade Organization rules.