After European Union leaders said they would keep negotiating instead of immediately retaliating against President Trump’s latest threat, businesses remain unable to develop long-term plans.

Even as European Union negotiators redoubled efforts to secure a trade deal with the United States, businesses in Europe face prolonged uncertainty that makes it difficult — and costly — for them to come up with plans.

The threat on Saturday from President Trump of a 30 percent tax on goods imported from the European Union leaves businesses facing tariff levels not seen since the end of the 19th century. Worse, many say, is that they find themselves in limbo, unable to make decisions on strategy, hiring and investments.

“Let’s be honest, an idea of 30 percent tariff rate is effectively prohibitive to the mutual trade,” Maros Sefcovic, the European Union’s trade commissioner, told reporters in Brussels on Monday. “The current uncertainty caused by unjustified tariffs cannot persist