Shares in Volkswagen plunged the most in almost six years in early Monday trading after U.S. authorities accused the German carmaker of falsifying emissions data, which means it could face penalties of up to $18 billion.
Europe's largest automaker is accused of designing software for diesel models of its core VW brand and luxury division Audi that deceives regulators measuring toxic emissions, the U.S. Environmental Protection Agency said on Friday.
VW shares fell 13 percent to 140.95 euros by 0207 EDT, the biggest one-day drop since November 2009.
The revelations have emerged just as VW was poised to shake off doubts about its leadership at a supervisory board meeting on Friday where it is aiming to take decisions on a new company structure and management lineup.
The German group is also grappling with operational issues such as falling sales in China and efforts to boost profitability at the troubled VW namesake brand.
"This disaster is beyond all expectations," Ferdinand Dudenhoeffer, head of the Center of Automotive Research at the University of Duisburg-Essen, said.
The emissions issue is plunging VW into more turmoil five months after former chairman Ferdinand Piech quit in a power struggle with Winterkorn over strategy.