Polestar chief executive Jim Brown has sensationally left the firm after only nine months at its helm.


In a statement, Polestar said he had resigned to "pursue opportunities outside of the Group".


Non-executive chairman Nicholas Bryan will take over as executive chairman on a full-time basis while Polestar searches for a replacement.


The Polestar board and major shareholder Investcorp thanked Brown for his "loyal service to the Group over many years".


The statement also said the group's goal was to involve all employees in the acceleration of its efficiency, capacity utilisation and customer service programmes "to achieve a performance that reflects the underlying potential of the Group".


Only last week, Brown said he was postive about Polestar's progress and gave no hints that his departure was imminent.


His resignation follows several weeks of controversy for the group after PrintWeek revealed news of Tony Rudston's 1.9m pay-off.


Brown took over as chief executive last May after Rudston "retired".


GPMU deputy general secretary Tony Burke said he was surprised by Brown's resignation.


"Two changes in nine months at the top of the UK's largest print company certainly gives us cause for concern. We were looking forward to a period of greater stability at Polestar following Tony Rudston's departure. The last thing Polestar needs is another period of uncertainty."


Burke said the union would seek a meeting with Bryan "as a matter of urgency" to discuss the group's future.