Audi’s technical development is being "comprehensively transformed", Mertens said, but he added: “This requires a high level of concentration, which I cannot fully manage in my situation. During this time, my health and my family have priority.
"I thank everyone at Audi for their commitment, their support and the great cooperation I have experienced during this time.”
Mertens’ replacement, Hans-Joachim Rothenpieler, joins from Volkswagen. He has lead the Volkswagen Group’s quality assurance division since February 2016, having joined the company in 1986.
Volkswagen Group boss Herbert Diess said: “We deeply regret that Peter Mertens is stepping down from the board of management. We respect the reasons that led to his decision and wish him a full recovery."
Diess continued: "Hans-Joachim Rothenpieler is an experienced expert. [He] has gained a great deal of experience in his numerous responsible positions at the Group, which will help him to make a start quickly in his new position at Audi.
"His task will be to continue the transformation of technical development and to further advance along the path towards electric mobility.”
Rothenpieler becomes the brand’s third development boss in just over two years, with Mertens having replaced Stefan Knirsch, who resigned in September 2016 after just nine months of service. His resignation followed an enquiry into his involvement in the Dieselgate emissions cheating scandal of 2016.