Vincent Bolloré has taken a 30% stake in a breakaway advertising agency formed by the founders of Publicis-owned Marcel, the agency that played a crucial role in securing for the network the lion's share of the £200m Orange business earlier this year.

The news is a major blow to Publicis and potentially threatens its hold on the Orange account it picked up in March.

Frederic Raillard and Farid Mokart, founders of Marcel, and Christophe Lambert, head of the French division of Publicis, have left to form the new agency, which is called FFL.

Mr Bolloré, the chairman of one of Publicis's rivals, Havas, and the largest shareholder in another, Aegis, has taken a 30% stake while the co-founders hold the remaining 70%.

In March, Publicis Groupe and Havas triumphed in Orange's high-profile, £200m, six-way international pitch.

The brief involved the rebranding of all of France Telecom's assets, which include Wanadoo in the UK, to Orange.

Publicis's Fallon office in London and Marcel in Paris were appointed to take the key roles for creating the consumer advertising across Europe.

Havas's Euro RSCG agency offices in London and Paris are responsible for Orange's business-to-business activities worldwide.

The defection of the leading executives follows last week's announcement by Publicis that Olivier Fleurot, the former chief executive of the Financial Times Group, had joined the agency as executive chairman.

Richard Pinder, president of the Leo Burnett agency network across the EMEA region, was promoted Mr Fleurot's second-in-command as chief operating officer.

· To contact the MediaGuardian newsdesk email editor@mediatheguardian.com or phone 020 7239 9857. For all other inquiries please call the main Guardian switchboard on 020 7278 2332.

· If you are writing a comment for publication, please mark clearly "for publication".