Vincent Bolloré, chairman and largest shareholder of French advertising group Havas, on Tuesday bought a further 11.3m shares in Aegis, taking his share of the UK advertising buyer from more than 25 per cent to 27.6 per cent.
The move will intensify speculation that Mr Bolloré, who has been gradually building his stake in Aegis over the last year, may make a takeover approach.
According to UK mergers and acquisitions rules, Mr Bolloré, who acquired the stake through his investment company Financiere du Loch, will have to make an offer for Aegis if his stake carries 30 per cent or more of the voting rights of the company.
Aegis shares rose 1.1 per cent per cent to 136¾p following the news. Havas was trading 0.5 per cent higher at €3.92.