Key French EADS shareholder Lagardere said the conditions attached to a planned merger of the European aerospace group with Britain's BAE Systems were unsatisfactory and called on management to reexamine the project.
A review is "indispensable" and should better take into account the interests of EADS' French shareholders, the company said in a statement on Monday.
"Despite the industrial and strategic potential attributed to it, this plan has not yet demonstrated that it was creating value for EADS," Lagardere said. "Lagardere considers that the merger conditions between EADS and BAE are currently unsatisfactory."
A spokesman for EADS declined to comment on the Lagardere statement on Monday.
EADS and BAE Systems announced last month that they were in talks to combine the companies to create what would be the world's biggest aerospace and defense group, marrying planemaker Airbus with Europe's largest defense contractor.
The bosses of both companies sought in a joint newspaper article published on Monday to reassure investors over the benefits of their planned $45 billion merger amid what they described as "myths and misconceptions" over the project.
EADS shares have fallen 17 percent since September 12, when word of the negotiations was leaked. After shedding initial gains, BAE shares are back to where they were beforehand.
EADS Chief Executive Tom Enders and BAE head Ian King argued that the plan to join forces was born out of opportunity, not necessity, and that it would create growth, being better able to ride the cycles of civil aviation demand and defense spending.
EADS was born from a merger of French, German and Spanish interests in 2000 with a unique structure allowing the Paris government and Daimler to cohabit at arm's length.
Lagardere, which wants to sell its EADS shareholding, has 7.5 percent but also represents the French government's 15 percent through the 12-year-old share pact. It shares control with Germany's Daimler , which owns a voting share of 22.5 percent of EADS.
(Reporting by James Regan; Editing by Blaise Robinson and Louise Heavens)