The Erlangen-based firm, which focuses on parabolic trough solar-thermal technology, has been dogged by accusations of irregular accounting practices in recent weeks, but denies all charges. Last month it pulled the curtains back its best-ever financial performance, having seen its revenues increase fivefold to €153.3m, while its operating profit nearly tripled to €31.1m.

Claassen, who served as chief executive of German power utility Energie Baden-Würrtemberg from 2003 to 2007, offered Solar Millenium no details on why he has stepped down.