The showdown takes place on Thursday in Paris in the House of Chemistry, at the back of the French national assembly, and the chemistry will certainly be explosive, more redolent of the boxing ring than of a revered industrial institution.

Alain de Pouzilhac, the chairman and chief executive of Havas, the world's sixth-largest advertising group, and Vincent Bolloré, the agency's biggest investor with 20% of the equity, will slug it out for effective control before hundreds of shareholders.

This year's annual meeting is the culmination of nine months of bitter feuding between the rival camps which, amid scarcely convincing protestations of innocence, have conducted their campaigns in the form of a media war that came to a head last weekend.

Yesterday de Pouzilhac, who has barely met his opponent and is no longer on speaking terms with him, took to Les Echos, the French financial daily, to deliver a withering attack on Bolloré's reputation as a business executive and as a man, accusing him of disloyalty and mendacity and of wanting to seize control of Havas by stealth and on the cheap before breaking it up and selling it on.

Forty-eight hours earlier, Bolloré had used the columns of Le Monde to denounce de Pouzilhac and his media advisers, their use of "notorious troublemakers and lobbyists" to destabilise Havas's biggest investor - and raise, for the first time, the prospect of the CEO's resignation and replacement.

The battle will come to a head over nominations to the 13-strong Havas board, with the outgoing team proposing three new directors to boost it to 16 and Bolloré nominating four, including himself and his nephew, to correspond with his investment vehicle's 20% stake.

Should, ironically, both sets of nominees be voted in by a simple majority then the board's strength of 20 would be illegal under French law, which requires a maximum 18 directors. Those with the biggest majorities would be elected.

Bolloré is regularly viewed by his critics as France's biggest corporate raider but has built up an industrial empire that employs 40,000 worldwide. With an annual turnover of more than €5bn (£3.4bn), his group, built around three family holdings, has become a global leader in transport services, plastic films and paper and operates rubber, palm and oil plantations.

In recent years, he has invested 10% of the group's wealth in the media, taking stakes in film distribution and production company Gaumont and launching a new TV channel in France. But Havas, where an initial 2% stake taken in July 2004 grew tenfold within less than three months, is his biggest media investment.

In his Le Monde interview, the Breton industrialist insisted that he was a long-term investor who had proved his commitment by taking part in the agency's €500m refinancing last year. "From the moment we joined, we declared that we did not want to take control of Havas. We are minority shareholders, we wish to hold between 20% and 30% of the capital and make our rights count ... There's no question of us making an offer for Havas."

But the sting in the tail of his proclamations that he is in for the long-term was the bald suggestion that de Pouzilhac might want to quit.

For de Pouzilhac this interview was the final proof that Bolloré is still refusing to answer basic questions about his intentions towards the group - questions that were first raised some six months ago.

De Pouzilhac and his aides recite a litany of occasions, dating back to Havas's abortive takeover approach for US rival Grey (acquired by WPP) last summer, when Bolloré said one thing and did another. He is accused of backing the Grey bid one day, opposing it another, of confiding with Havas's rivals and of trying to hold confidential meetings with six top managers.

He is accused of undermining the agency's tenders for key contracts through his destabilising tactics and of acting behind de Pouzilhac's back.

De Pouzilhac's biggest weapon is the financial turnaround at Havas which lost a net €389m in 2003 and made €43m in 2004. He can also count on support from small shareholders' action groups such as ADAM and advisers such as ISS and Proxinvest which oppose Bolloré. Even Nicholas Miguet, the rebel shareholder who organised last year's coup at Eurotunnel, has been brought on-side.

But de Pouzilhac does not seem as confident of assembling a 20% counter-bloc to Bolloré's as he once was, leaving the outcome of Thursday's head-to-head uncertain.

"The AGM is sovereign and I bow to it. It's the one that decides and the more voters there are, the better for shareholder democracy."