Qatar Holding said in the filing that it could raise its stake further after increasing it to 12.8 percent and that it may seek representation on the group's supervisory board.
Qatar Holding added that it may discuss strategic partnerships "allowing the creation of long-term value for shareholders" of Lagardere, though there were currently no specific plans, Reuters reports.
Lagardere shares were roughly flat at 23.565 euros at 1006 GMT.
The company had no immediate comment.
Qatar has been a staunch supporter of Chief Executive Arnaud Lagardere even as some shareholders have assailed the chief executive's strategy for the company his father founded, which competes with Pearson and Bertelsmann in radio and book publishing.
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