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Last year, the French government said that it planned to inject €9bn (£5.9bn) into the struggling telecoms operator.
The Commission spokesman on state aid matters, Tilman Luder, said that the decision on whether the injection constitutes aid is difficult, owing to the complexity of the transaction, but he indicated that a formal investigation was likely because only then could third parties, such as rival telecom companies, contribute comments to help the commission's assessment.
"It's a difficult and a big case," Luder said. "The opinions of competitors and others are sought."
Telecoms ministers from other European Union countries have spoken out against the aid.