Mills had assumed responsibility for the web offset printer after former managing director William Mills left at the end of 2001.
He has been involved in negotiations with the GPMU about a restructuring programme.
His departure has come as a huge surprise. Mills was the first non-family member to lead the 70m group and brought fresh thinking to the company. However, it's thought that some aspects of his planned change programme may have unsettled Jarrold family members.
Charles Jarrold said: "It is our firm intention to develop further the current business strategy and modernisation programme established under the leadership of Rob Mills."
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