Richard Barber, Quicks group chief executive, resigned last night. A company spokesman, who gave no reason, said Mr Barber had immediately cleared his desk and left.
Former Lex Service executive David Beck has joined the Quicks board and will be acting chief executive until a replacement for Mr Barber is appointed.
Analysts believe other directors had become disillusioned with Quicks' performance under Mr Barber's leadership.
Mr Barber was appointed managing director of Quicks' motor retail group at the time of the Caverdale acquisition in 1997.
He was appointed group chief executive in January 1999, when Mr Murray was appointed deputy chairman.
Quicks is the UK's 15th largest motor dealer group with a turnover of £570m. It is set to announce its annual results next Friday (Mar 30).