Shares in German car maker Volkswagen have plunged by a fifth after it admitted rigging US emissions tests for nearly 500,000 diesel cars and was ordered to recall the vehicles to be fixed.
Chief executive Marin Winterkorn said he was "deeply sorry" after findings by America's Environmental Protection Agency (EPA) that it had cheated clean-air rules. It faces fines that could total billions of dollars.
The EPA said VW used software that allowed its diesel cars to release fewer smog-causing pollutants during tests than in real-world driving conditions.
Its findings cover 482,000 cars built in the last seven years including the Audi A3, VW Jetta, Beetle, Golf and Passat models.
The EPA said cars had been fitted with sophisticated software algorithms which detect when they are undergoing officials emissions testing, and turns full emissions controls on only reducing the test. It is a type of software known as a "defeat device".
Once on the road, the cars produced nitrogen oxide pollutants at up to 40 times the legal standard.
Cynthia Giles, assistant administrator for the Office of Enforcement and Compliance Assurance, said: "Using a defeat device in cars to evade clean air standards is illegal and a threat to public health."
Mr Winterkorn said: "I personally am deeply sorry that we have broken the trust of our customers and the public.
"We will co-operate fully with the responsible agencies, with transparency and urgency, to clearly, openly, and completely establish all of the facts of this case. Volkswagen has ordered an external investigation of this matter.
"We do not and will not tolerate violations of any kind of our internal rules or of the law."
The EPA said VW faces fines of up to 37,500 US dollars (£24,000) per vehicle for the violations - a total of more than 18 billion US dollars (£12 billion).
It said VW might need up to a year to carry out the recall.
The shares fall wiped around 15 billion euros (£11 billion) off VW's market value, as the stock reached a three-year low of around 130 euros (£95).
It has already had a tough year having fallen from more than 250 euros (£180) amid signs of faltering sales in the US and China.
VW edged out Toyota to become the world's top-selling car maker in the first half of 2015.