The United States, as the second-largest car market after China, is crucial to Volkswagen’s long-term strategy. But it accounts for only 6 percent of unit sales, compared with 40 percent in Europe and Russia. Problems in Europe are potentially a far greater threat to Volkswagen’s financial strength and its ability to invest in new technologies that are expected to transform the industry in years to come.

Revelations that Volkswagen cars may not be as environmentally responsible as its “clean diesel” advertising had promised are potentially even more damaging to the company’s image in Europe than in the United States. Diesel vehicles account for more than half of all vehicles sold in Europe largely because of government policies that have made diesel fuels cheaper than gasoline, and because of less stringent emissions standards for diesels than in the United States.

The carmaker’s announcement on Tuesday was its first admission that diesel cars outside the United States contained the software that led the Environmental Protection Agency to accuse the company of deliberately evading pollution tests. Previously, Volkswagen had acknowledged only that the problem affected about 500,000 vehicles in the United States.

Volkswagen’s majority shareholder, Porsche Holding, was silent on Mr. Winterkorn on Tuesday, but he faces questions by a subcommittee of the company’s supervisory board on Wednesday.

Earlier this year, Mr. Winterkorn had the support of most of the Porsche family in a power struggle with Ferdinand Piëch, the company’s chairman at the time. Mr. Winterkorn prevailed and Mr. Piëch resigned. But the family derives much of its wealth from Volkswagen and is said to be watching the diesel scandal with concern.