European Union regulators ramped up their investigation of the Chinese solar panel industry on Thursday by accusing the Chinese government of unfairly subsidizing manufacturers of the panels.

The latest step in an increasingly acrimonious battle engulfing the clean-energy sector came a day after the United States made a final decision to impose duties on billions of dollars of solar products from China over the next five years to shield producers against lower-priced imports.

It also came after the government in Beijing said Monday that it had filed a case with the World Trade Organization accusing some E.U. countries of violating free trade rules with policies that favored the purchase of solar energy equipment produced in Europe.