Loss-making Lagardere has become the unlikely target of a proxy battle between some of France's top businessmen, after the firm's managers sought help to fend off a campaign by activist investor Amber Capital to shake up the company's governance.
New investors came on board as a result, including Vincent Bollore, another French billionaire who controls media firm Vivendi.
But the new array of shareholders are now at odds over Lagardere's strategy, and the spat has fuelled speculation that Bollore and Arnault, who runs luxury goods conglomerate LVMH, will want to vie for some of its assets, which include publishing house Hachette and a travel retail division.
Arnault said on Thursday that his family holding company now owned more than 5% of Lagardere, and that he was committed to the integrity of the group's main activities.
Arnault has also invested in the vehicle through which Lagardere's Chief Executive, Arnaud Lagardere, controls the firm and said the two, with a 12.8% holding between them, would act in concert.
While Vivendi has recently built up a 23% stake, and has teamed up with Amber, a 20% stakeholder, in a bid to be awarded board seats, Arnaud Lagardere exerts great sway over the company as his stake is held through an unusual "commandite" structure that gives him veto powers on many decisions.
A spokesman for Vivendi said on Thursday the firm was "very happy" that Bernard Arnault had become a direct Lagardere shareholder.
Bollore initially rode to Arnaud Lagardere's rescue earlier this year to help him fight off Amber, but the mood soured after it emerged Bernard Arnault would also get involved, people close to the matter have said.
Vivendi and Amber have now gone to court to try and trigger a swift shareholder meeting to change the group's governance. Its supervisory board unexpectedly renewed Arnaud Lagardere's mandate as CEO months ahead of schedule in August.
"It's a company that's managed only in the interest of one person and those close to him," Amber's managing partner Joseph Oughourlian said on the sidelines of a court hearing on the case on Thursday.
Lagardere's lawyers Florian Bouaziz and Didier Malka brushed off accusations of a lack of independence of the board, saying that Vivendi had initially voted for the renewal of the current one and against Amber.
The court will rule on the shareholder meeting request on Oct. 14.
By Sarah White and Mathieu Rosemain