Bollore has increased his share of the communications group ranked sixth in the world from 18 to 20.2 per cent, fuelling speculation that it is being set up for a takeover either by WPP or by Publicis Groupe.
French law dictates that any shareholder whose stake reaches 33 per cent must proceed with a takeover bid.
Havas bosses are viewing Bollore's increasing influence with alarm. In an interview with the newspaper Le Figaro, Alain de Pouzilhac, the Havas chief executive, urged Bollore to declare his intentions.
"Given his track record, you have to admit that our questions are legitimate," he said.
Although he said he had plans to increase his stake still further, Bollore has denied working behind the scenes to orchestrate a takeover. He claimed he would be appointing directors to the Havas administrative board as a sign of his "long-term commitment" to the company.
As the last medium-sized player in an industry dominated by four big groups - WPP, Publicis, Omnicom and Interpublic - Havas is seen as increasingly ripe for a takeover.
Its position has looked even more vulnerable since it missed out to WPP in the bidding war for Grey Global Group and it is also facing the possible loss of its £168 million global Intel account.
Publicis sources warn that any deal would be made difficult by huge conflict problems and the fact that the two groups have been bitter rivals for many years.
"We're old adversaries who have fought for the same business for a long time," a Publicis source said.
- Close-up, p18.
This article was first published on Campaign