"It is my belief that, while hoping for the best, we must be ready to prepare our Union for worst-case scenarios," Tusk wrote in a letter to EU leaders.
Last Friday, US President Donald Trump said in a statement that if the European Union did not remove tariffs and trade barriers against the United States soon, his administration would place a 20 percent tariff on all cars coming from the EU.
READ MORE: 'Trade Must Be Fair': Trump Calls on All Countries to Remove Existing Tariffs
Trade disagreements between the US and the EU have intensified since Washington imposed a 25 percent tariff on imported steel and a 10 percent tariff on aluminum imports.
Transatlantic relations hit their lowest point after Donald Trump blamed his partners and allies for taking advantage of the US in terms of trade following the recent G7 summit.