Volkswagen Chief Executive Martin Winterkorn speaks at the annual news conference of Volkswagen in Berlin March 12, 2015. REUTERS/Fabrizio Bensch BERLIN (Reuters) - Former Volkswagen Chief Executive Martin Winterkorn has stepped down as chairman of luxury brand Audi amid an investigation of two emissions scandals that occurred during his reign at Europe's largest carmaker.

Winterkorn quit his post at Audi on Wednesday, a spokesman at the Ingolstadt-based carmaker said on Thursday, after resigning as chief executive of family-owned Porsche SE, VW's majority stakeholder, last month.

The 68-year-old Winterkorn was forced to resign as VW group chief executive by the carmaker's influential labor leaders and the state of Lower Saxony, VW's No. 2 shareholder, on Sept. 23, five days after the company's rigging of diesel emissions tests became public in the United States.

VW has been rocked by revelations that its diesel cars were equipped with a software capable of cheating emissions rules, affecting 11 million vehicles worldwide including Audi models.

The scandal took on a new dimension last week when Wolfsburg-based VW admitted it also manipulated carbon dioxide emissions levels of 800,000 cars, adding this could cost it another 2 billion euros ($2.15 billion) besides the 6.7 billion it has set aside to cover costs related to the diesel-emissions fiasco.

A successor to Winterkorn as Audi chairman has yet to be named, the spokesman said.

(Reporting by Andreas Cremer; editing by Jason Neely)