EADS shareholder Lagardere has told the aerospace group to review the "unsatisfactory" terms of its planned $45-billion merger with BAE Systems.
The development raises the pressure before a British regulatory deadline next week.
The French firm's demand marks another setback to the politically sensitive plan to create a European defence and aerospace giant before next week's deadline.
Some market-watchers suggested Lagardere was simply seeking better conditions for a future sale of its minority stake in EADS rather than fundamentally opposing the deal with the British group.
"Despite the industrial and strategic potential attributed to it, this plan has not yet demonstrated that it was creating value for EADS," Lagardere said in a statement.
Lagardere expressed dissatisfaction with the terms of the deal, under which EADS is currently due to get around 60% of the combined company.
"Lagardere considers that the merger conditions between EADS and BAE are currently unsatisfactory," it said.
Barring an extension, EADS and BAE have until Wednesday next week under UK takeover rules to set out detailed plans for the merger.
Lagardere holds 7.5% of EADS, half the French government's 15%, but is the senior partner on the French side of the Franco-German shareholder pact which underpins the group.
Germany is represented by carmaker Daimler's 22.5% voting share.
EADS and BAE Systems announced last month they were in talks to form what would be the world's biggest aerospace and defence group, marrying planemaker Airbus with Europe's largest defence contractor.
