Ulrich Lehner, 72, had publicly supported Heinrich Hiesinger, who resigned unexpectedly earlier this month following a merger of Thyssenkrupp's steelmaking business with India's Tata, creating Europe's second biggest steelmaker.

Thyssenkrupp has been under pressure from activist investors such as investment firm Cevian, an 18 percent shareholder, and business daily Handelsblatt reported Lehner was one of two advisory board members to have voted against the Tata merger.

Lehner told German press last week that activist shareholders had inflicted "psychological terrorism" on Hiesinger.