Former Volkswagen chief executive Martin Winterkorn has stepped down as chairman of the group's luxury brand Audi amid an investigation of two emissions scandals that occurred during his reign at Europe’s largest car maker.

Winterkorn quit his post at Audi on Wednesday, a spokesman told Reuters and other news agencies on Thursday, after resigning as chief executive of family-owned Porsche SE, VW’s majority stakeholder, last month.

Winterkorn was forced to resign as VW group chief executive by the car maker’s influential labour leaders and the state of Lower Saxony, VW’s second largest shareholder, on 23 September, five days after the company’s rigging of diesel emissions tests became public in the United States.

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