Shares in German auto giant Volkswagen fell more than 13 percent on the Frankfurt stock exchange on Monday after it emerged some of its diesel cars in the United States had been fitted with software that gave false emissions data.
In the first few minutes of trading, VW shares fell by as much as 13.23 percent to 140 euros in a slightly lower market.
In a statement on Sunday, the carmaker had said that the US Environmental Protection Agency and the California Air Resources Board (EPA and CARB) had found that while testing diesel cars of the Volkswagen Group they had "detected manipulations that violate American environmental standards."
VW chief executive Martin Winterkorn issued an apology and said the group had ordered an external investigation into the matter.
"The board of management takes these findings very seriously. I personally am deeply sorry that we have broken the trust of our customers and the public. We will cooperate fully with the responsible agencies, with transparency and urgency, to clearly, openly and completely establish all of the facts of this case," Winterkorn said.
"We at Volkswagen will do everything that must be done in order to re-establish the trust that so many people have placed in us, and we will do everything necessary in order to reverse the damage this has caused. This matter has first priority for me, personally, and for our entire board of management," Winterkorn said.