Bollore, who took a stand against Havas' decision to bid for Grey Global Group, now has five days to declare his intentions for the company.
At the same time, Alain de Pouzilhac, the Havas chairman, has come under increasing pressure following the failure of the network's bid for Grey.
De Pouzilhac led the £900 million-plus bid, which also lacked the support of the Havas house bank, Societe Generale, which has a seat on the company's board. Some shareholders are angry that their chairman chose to pursue a strategy they considered risky and overly ambitious.
He is already feeling the pressure because of the company's recent poor financial performance. Havas, which owns the Euro RSCG and Media Planning Group networks, posted a net loss of £271 million last year.
De Pouzilhac saw Grey as Havas' last chance to achieve critical mass.
He said: "When an opportunity like Grey comes to the market, it is our duty to assess whether it can significantly add value to our group."
The bid's failure now leaves Havas vulnerable to takeover, with WPP and Publicis the most likely predators.