Shares in Volkswagen (VOWG_p.DE) 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.