The move is part of a drive to boost profitability and organic growth, which has suffered as a result of an advertising freeze following the WorldCom accounting scandal in the summer.
The chairman and CEO of Euro RSCG Worldwide is working increasingly closely with de Pouzilhac to form a new team that will be known as "the CEO's office".
According to de Pouzilhac: "I've known Bob for a long time and we have developed a strong friendship and trust. We share the same values, the same vision and view of our times, and we have the same ambition for Havas."
Last month, the company said revenues dropped nearly 6% to €443m (£285m) for the third quarter, due to the collapse of WorldCom.
In a statement, it said that organic growth had been "badly hit (around -1%) by the virtual halt in WorldCom advertising expenditure during that period, after the disclosure of its accounting irregularities".
However, Havas added that it should not impact on fourth-quarter revenues because WorldCom had subsequently resumed advertising.
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